TY  - JOUR
A1  - Willig, Lisa
A1  - Reppert, Alexander von
A1  - Deb, Marwan
A1  - Ganss, F.
A1  - Hellwig, O.
A1  - Bargheer, Matias
T1  - Finite-size effects in ultrafast remagnetization dynamics of FePt
JF  - Physical review : B, Condensed matter and materials physics
N2  - We investigate the ultrafast magnetization dynamics of FePt in the L1(0) phase after an optical heating pulse, as used in heat-assisted magnetic recording. We compare continuous and nano-granular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain-wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials.
Y1  - 2019
U6  - https://doi.org/10.1103/PhysRevB.100.224408
SN  - 2469-9950
SN  - 2469-9969
VL  - 100
IS  - 22
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Warby, Jonathan
A1  - Zu, Fengshuo
A1  - Zeiske, Stefan
A1  - Gutierrez-Partida, Emilio
A1  - Frohloff, Lennart
A1  - Kahmann, Simon
A1  - Frohna, Kyle
A1  - Mosconi, Edoardo
A1  - Radicchi, Eros
A1  - Lang, Felix
A1  - Shah, Sahil
A1  - Pena-Camargo, Francisco
A1  - Hempel, Hannes
A1  - Unold, Thomas
A1  - Koch, Norbert
A1  - Armin, Ardalan
A1  - De Angelis, Filippo
A1  - Stranks, Samuel D.
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
T1  - Understanding performance limiting interfacial recombination in pin Perovskite solar cells
JF  - Advanced energy materials
N2  - Perovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto-electronic properties and their successful integration into multijunction cells. However, the performance of single- and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C-60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first-principle numerical simulations. It is found that the most significant contribution to the total C-60-induced recombination loss occurs within the first monolayer of C-60, rather than in the bulk of C-60 or at the perovskite surface. The experiments show that the C-60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C-60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells.
KW  - C60
KW  - defects
KW  - interface recombination
KW  - loss mechanisms
KW  - perovskites
KW  - solar cells
Y1  - 2022
U6  - https://doi.org/10.1002/aenm.202103567
SN  - 1614-6832
SN  - 1614-6840
VL  - 12
IS  - 12
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Abdalla, Hassan E.
A1  - Adam, R.
A1  - Aharonian, Felix A.
A1  - Benkhali, F. Ait
A1  - Angüner, Ekrem Oǧuzhan
A1  - Arakawa, M.
A1  - Arcaro, C.
A1  - Armand, C.
A1  - Ashkar, H.
A1  - Backes, M.
A1  - Martins, V. Barbosa
A1  - Barnard, M.
A1  - Becherini, Y.
A1  - Berge, D.
A1  - Bernloehr, K.
A1  - Blackwell, R.
A1  - Böttcher, M.
A1  - Boisson, C.
A1  - Bolmont, J.
A1  - Bonnefoy, S.
A1  - Bregeon, J.
A1  - Breuhaus, M.
A1  - Brun, F.
A1  - Brun, P.
A1  - Bryan, M.
A1  - Büchele, M.
A1  - Bulik, T.
A1  - Bylund, T.
A1  - Capasso, M.
A1  - Caroff, S.
A1  - Carosi, A.
A1  - Casanova, Sabrina
A1  - Cerruti, M.
A1  - Chand, T.
A1  - Chandra, S.
A1  - Chen, A.
A1  - Colafrancesco, S.
A1  - Curylo, M.
A1  - Davids, I. D.
A1  - Deil, C.
A1  - Devin, J.
A1  - DeWilt, P.
A1  - Dirson, L.
A1  - Djannati-Ata, A.
A1  - Dmytriiev, A.
A1  - Donath, A.
A1  - Doroshenko, V
A1  - Dyks, J.
A1  - Egberts, Kathrin
A1  - Emery, G.
A1  - Ernenwein, J-P
A1  - Eschbach, S.
A1  - Feijen, K.
A1  - Fegan, S.
A1  - Fiasson, A.
A1  - Fontaine, G.
A1  - Funk, S.
A1  - Füßling, Matthias
A1  - Gabici, S.
A1  - Gallant, Y. A.
A1  - Gate, F.
A1  - Giavitto, G.
A1  - Glawion, D.
A1  - Glicenstein, J. F.
A1  - Gottschall, D.
A1  - Grondin, M-H
A1  - Hahn, J.
A1  - Haupt, M.
A1  - Heinzelmann, G.
A1  - Henri, G.
A1  - Hermann, G.
A1  - Hinton, James Anthony
A1  - Hofmann, W.
A1  - Hoischen, Clemens
A1  - Holch, Tim Lukas
A1  - Holler, M.
A1  - Horns, D.
A1  - Huber, D.
A1  - Iwasaki, H.
A1  - Jamrozy, M.
A1  - Jankowsky, D.
A1  - Jankowsky, F.
A1  - Jardin-Blicq, A.
A1  - Jung-Richardt, I
A1  - Kastendieck, M. A.
A1  - Katarzynski, K.
A1  - Katsuragawa, M.
A1  - Katz, U.
A1  - Khangulyan, D.
A1  - Khelifi, B.
A1  - King, J.
A1  - Klepser, S.
A1  - Kluzniak, W.
A1  - Komin, Nu
A1  - Kosack, K.
A1  - Kostunin, D.
A1  - Kraus, M.
A1  - Lamanna, G.
A1  - Lau, J.
A1  - Lemiere, A.
A1  - Lemoine-Goumard, M.
A1  - Lenain, J-P
A1  - Leser, Eva
A1  - Levy, C.
A1  - Lohse, T.
A1  - Lypova, I
A1  - Mackey, J.
A1  - Majumdar, J.
A1  - Malyshev, D.
A1  - Marandon, V
A1  - Marcowith, Alexandre
A1  - Mares, A.
A1  - Mariaud, C.
A1  - Marti-Devesa, G.
A1  - Marx, R.
A1  - Maurin, G.
A1  - Meintjes, P. J.
A1  - Mitchell, A. M. W.
A1  - Moderski, R.
A1  - Mohamed, M.
A1  - Mohrmann, L.
A1  - Moore, C.
A1  - Moulin, Emmanuel
A1  - Muller, J.
A1  - Murach, T.
A1  - Nakashima, S.
A1  - de Naurois, M.
A1  - Ndiyavala, H.
A1  - Niederwanger, F.
A1  - Niemiec, J.
A1  - Oakes, L.
A1  - Odaka, H.
A1  - Ohm, S.
A1  - Wilhelmi, E. de Ona
A1  - Ostrowski, M.
A1  - Oya, I
A1  - Panter, M.
A1  - Parsons, R. D.
A1  - Perennes, C.
A1  - Petrucci, P-O
A1  - Peyaud, B.
A1  - Piel, Q.
A1  - Pita, S.
A1  - Poireau, V
A1  - Priyana Noel, A.
A1  - Prokhorov, D. A.
A1  - Prokoph, H.
A1  - Pühlhofer, G.
A1  - Punch, M.
A1  - Quirrenbach, A.
A1  - Raab, S.
A1  - Rauth, R.
A1  - Reimer, A.
A1  - Reimer, O.
A1  - Remy, Q.
A1  - Renaud, M.
A1  - Rieger, F.
A1  - Rinchiuso, L.
A1  - Romoli, C.
A1  - Rowell, G.
A1  - Rudak, B.
A1  - Ruiz-Velasco, E.
A1  - Sahakian, V
A1  - Saito, S.
A1  - Sanchez, David M.
A1  - Santangelo, Andrea
A1  - Sasaki, M.
A1  - Schlickeiser, R.
A1  - Schüssler, F.
A1  - Schulz, A.
A1  - Schutte, H.
A1  - Schwanke, U.
A1  - Schwemmer, S.
A1  - Seglar-Arroyo, M.
A1  - Senniappan, M.
A1  - Seyffert, A. S.
A1  - Shafi, N.
A1  - Shiningayamwe, K.
A1  - Simoni, R.
A1  - Sinha, A.
A1  - Sol, H.
A1  - Specovius, A.
A1  - Spir-Jacob, M.
A1  - Stawarz, L.
A1  - Steenkamp, R.
A1  - Stegmann, Christian
A1  - Steppa, Constantin Beverly
A1  - Takahashi, T.
A1  - Tavernier, T.
A1  - Taylor, A. M.
A1  - Terrier, R.
A1  - Tiziani, D.
A1  - Tluczykont, M.
A1  - Trichard, C.
A1  - Tsirou, M.
A1  - Tsuji, N.
A1  - Tuffs, R.
A1  - Uchiyama, Y.
A1  - van Der Walt, D. J.
A1  - van Eldik, C.
A1  - van Rensburg, C.
A1  - van Soelen, B.
A1  - Vasileiadis, G.
A1  - Veh, J.
A1  - Venter, C.
A1  - Vincent, P.
A1  - Vink, J.
A1  - Voisin, F.
A1  - Voelk, H. J.
A1  - Vuillaume, T.
A1  - Wadiasingh, Z.
A1  - Wagner, S. J.
A1  - White, R.
A1  - Wierzcholska, A.
A1  - Yang, R.
A1  - Yoneda, H.
A1  - Zacharias, Michael
A1  - Zanin, R.
A1  - Zdziarski, A. A.
A1  - Zech, Alraune
A1  - Ziegler, A.
A1  - Zorn, J.
A1  - Zywucka, N.
A1  - Meyer, M.
T1  - Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS
JF  - Astronomy and astrophysics : an international weekly journal
N2  - The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100MeV < E < 100 GeV) gamma-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E > 100 GeV) gamma-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8 : 7 sigma significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 +/- 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r greater than or similar to 1 : 7 X 1017 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279.
KW  - radiation mechanisms: non-thermal
KW  - quasars: individual: 3C 279
KW  - galaxies: active
KW  - relativistic processes
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201935704
SN  - 1432-0746
VL  - 627
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Abdalla, Hassan E.
A1  - Collaboration, H. E. S. S.
A1  - Abramowski, A.
A1  - Aharonian, Felix A.
A1  - Benkhali, F. Ait
A1  - Angüner, Ekrem Oǧuzhan
A1  - Arakawa, M.
A1  - Armand, C.
A1  - Arrieta, M.
A1  - Backes, M.
A1  - Balzer, A.
A1  - Barnard, M.
A1  - Becherini, Y.
A1  - Tjus, J. Becker
A1  - Berge, D.
A1  - Bernhard, S.
A1  - Bernloehr, K.
A1  - Blackwell, R.
A1  - Bottcher, M.
A1  - Boisson, C.
A1  - Bolmont, J.
A1  - Bonnefoy, S.
A1  - Bordas, Pol
A1  - Bregeon, J.
A1  - Brun, F.
A1  - Brun, P.
A1  - Bryan, M.
A1  - Buechele, M.
A1  - Bulik, T.
A1  - Capasso, M.
A1  - Caroff, S.
A1  - Carosi, A.
A1  - Casanova, Sabrina
A1  - Cerruti, M.
A1  - Chakraborty, N.
A1  - Chaves, R. C. G.
A1  - Chen, A.
A1  - Chevalier, J.
A1  - Colafrancesco, S.
A1  - Condon, B.
A1  - Conrad, J.
A1  - Davids, I. D.
A1  - Decock, J.
A1  - Deil, C.
A1  - Devin, J.
A1  - deWilt, P.
A1  - Dirson, L.
A1  - Djannati-Atai, A.
A1  - Donath, A.
A1  - Dyks, J.
A1  - Edwards, T.
A1  - Egberts, Kathrin
A1  - Emery, G.
A1  - Ernenwein, J. -P.
A1  - Eschbach, S.
A1  - Farnier, C.
A1  - Fegan, S.
A1  - Fernandes, M. V.
A1  - Fiasson, A.
A1  - Fontaine, G.
A1  - Funk, S.
A1  - Fuessling, M.
A1  - Gabici, S.
A1  - Gallant, Y. A.
A1  - Garrigoux, T.
A1  - Gate, F.
A1  - Giavitto, G.
A1  - Glawion, D.
A1  - Glicenstein, J. F.
A1  - Gottschall, D.
A1  - Grondin, M. -H.
A1  - Hahn, J.
A1  - Haupt, M.
A1  - Hawkes, J.
A1  - Heinzelmann, G.
A1  - Henri, G.
A1  - Hermann, G.
A1  - Hinton, J. A.
A1  - Hofmann, W.
A1  - Hoischen, Clemens
A1  - Holch, T. L.
A1  - Holler, M.
A1  - Horns, D.
A1  - Ivascenko, A.
A1  - Iwasaki, H.
A1  - Jacholkowska, A.
A1  - Jamrozy, M.
A1  - Jankowsky, D.
A1  - Jankowsky, F.
A1  - Jingo, M.
A1  - Jouvin, L.
A1  - Jung-Richardt, I.
A1  - Kastendieck, M. A.
A1  - Katarzynski, K.
A1  - Katsuragawa, M.
A1  - Katz, U.
A1  - Kerszberg, D.
A1  - Khangulyan, D.
A1  - Khelifi, B.
A1  - King, J.
A1  - Klepser, S.
A1  - Klochkov, D.
A1  - Kluzniak, W.
A1  - Komin, Nu.
A1  - Kosack, K.
A1  - Krakau, S.
A1  - Kraus, M.
A1  - Kruger, P. P.
A1  - Laffon, H.
A1  - Lamanna, G.
A1  - Lau, J.
A1  - Lefaucheur, J.
A1  - Lemiere, A.
A1  - Lemoine-Goumard, M.
A1  - Lenain, J. -P.
A1  - Leser, Eva
A1  - Lohse, T.
A1  - Lorentz, M.
A1  - Liu, R.
A1  - Lopez-Coto, R.
A1  - Lypova, I.
A1  - Malyshev, D.
A1  - Marandon, V.
A1  - Marcowith, Alexandre
A1  - Mariaud, C.
A1  - Marx, R.
A1  - Maurin, G.
A1  - Maxted, N.
A1  - Mayer, M.
A1  - Meintjes, P. J.
A1  - Meyer, M.
A1  - Mitchell, A. M. W.
A1  - Moderski, R.
A1  - Mohamed, M.
A1  - Mohrmann, L.
A1  - Mora, K.
A1  - Moulin, Emmanuel
A1  - Murach, T.
A1  - Nakashima, S.
A1  - de Naurois, M.
A1  - Ndiyavala, H.
A1  - Niederwanger, F.
A1  - Niemiec, J.
A1  - Oakes, L.
A1  - Odaka, H.
A1  - Ohm, S.
A1  - Ostrowski, M.
A1  - Oya, I.
A1  - Padovani, M.
A1  - Panter, M.
A1  - Parsons, R. D.
A1  - Pekeur, N. W.
A1  - Pelletier, G.
A1  - Perennes, C.
A1  - Petrucci, P. -O.
A1  - Peyaud, B.
A1  - Piel, Q.
A1  - Pita, S.
A1  - Poireau, V.
A1  - Prokhorov, D. A.
A1  - Prokoph, H.
A1  - Puehlhofer, G.
A1  - Punch, M.
A1  - Quirrenbach, A.
A1  - Raab, S.
A1  - Rauth, R.
A1  - Reimer, A.
A1  - Reimer, O.
A1  - Renaud, M.
A1  - de los Reyes, R.
A1  - Rieger, F.
A1  - Rinchiuso, L.
A1  - Romoli, C.
A1  - Rowell, G.
A1  - Rudak, B.
A1  - Rulten, C. B.
A1  - Sahakian, V.
A1  - Saito, S.
A1  - Sanchez, D. A.
A1  - Santangelo, Andrea
A1  - Sasaki, M.
A1  - Schlickeiser, R.
A1  - Schussler, F.
A1  - Schulz, A.
A1  - Schwanke, U.
A1  - Schwemmer, S.
A1  - Seglar-Arroyo, M.
A1  - Seyffert, A. S.
A1  - Shafi, N.
A1  - Shilon, I.
A1  - Shiningayamwe, K.
A1  - Simoni, R.
A1  - Sol, H.
A1  - Spanier, F.
A1  - Spir-Jacob, M.
A1  - Stawarz, L.
A1  - Steenkamp, R.
A1  - Stegmann, Christian
A1  - Steppa, Constantin Beverly
A1  - Sushch, I.
A1  - Takahashi, T.
A1  - Tavernet, J. -P.
A1  - Tavernier, T.
A1  - Taylor, A. M.
A1  - Terrier, R.
A1  - Tibaldo, L.
A1  - Tiziani, D.
A1  - Tluczykont, M.
A1  - Trichard, C.
A1  - Tsirou, M.
A1  - Tsuji, N.
A1  - Tuffs, R.
A1  - Uchiyama, Y.
A1  - van der Walt, D. J.
A1  - van Eldik, C.
A1  - van Rensburg, C.
A1  - van Soelen, B.
A1  - Vasileiadis, G.
A1  - Veh, J.
A1  - Venter, C.
A1  - Viana, A.
A1  - Vincent, P.
A1  - Vink, J.
A1  - Voisin, F.
A1  - Voelk, H. J.
A1  - Vuillaume, T.
A1  - Wadiasingh, Z.
A1  - Wagner, S. J.
A1  - Wagner, P.
A1  - Wagner, R. M.
A1  - White, R.
A1  - Wierzcholska, A.
A1  - Willmann, P.
A1  - Woernlein, A.
A1  - Wouters, D.
A1  - Yang, R.
A1  - Zaborov, D.
A1  - Zacharias, M.
A1  - Zanin, R.
A1  - Zdziarski, A. A.
A1  - Zech, Alraune
A1  - Zefi, F.
A1  - Ziegler, A.
A1  - Zorn, J.
A1  - Zywucka, N.
T1  - Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Recently, the high-energy (HE, 0.1-100 GeV) gamma-ray emission from the object LMC P3 in the Large Magellanic Cloud (LMC) has been discovered to be modulated with a 10.3-day period, making it the first extra-galactic gamma-ray binary. Aims. This work aims at the detection of very-high-energy (VHE, >100 GeV) gamma-ray emission and the search for modulation of the VHE signal with the orbital period of the binary system. Methods. LMC P3 has been observed with the High Energy Stereoscopic System (H.E.S.S.); the acceptance-corrected exposure time is 100 h. The data set has been folded with the known orbital period of the system in order to test for variability of the emission. Results. VHE gamma-ray emission is detected with a statistical significance of 6.4 sigma. The data clearly show variability which is phase-locked to the orbital period of the system. Periodicity cannot be deduced from the H.E.S.S. data set alone. The orbit-averaged luminosity in the 1-10 TeV energy range is (1.4 +/- 0.2) x 10(35) erg s(-1). A luminosity of (5 +/- 1) x 10(35) erg s(-1) is reached during 20% of the orbit. HE and VHE gamma-ray emissions are anti-correlated. LMC P3 is the most luminous gamma-ray binary known so far.
KW  - gamma rays: stars
KW  - binaries: general
KW  - stars: massive
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201732426
SN  - 1432-0746
VL  - 610
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Schwope, Axel
A1  - Pires, Adriana M.
A1  - Kurpas, Jan
A1  - Doroshenko, Victor
A1  - Suleimanov, Valery F.
A1  - Freyberg, Michael
A1  - Becker, Werner
A1  - Dennerl, Konrad
A1  - Haberl, Frank
A1  - Lamer, Georg
A1  - Maitra, Chandreyee
A1  - Potekhin, Alexander Y.
A1  - Ramos-Ceja, Miriam E.
A1  - Santangelo, Andrea
A1  - Traulsen, Iris
A1  - Werner, Klaus
T1  - Phase-resolved X-ray spectroscopy of PSR B0656+14 with SRG/eROSITA and XMM-Newton
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We present a detailed spectroscopic and timing analysis of X-ray observations of the bright pulsar PSR B0656+14. The observations were obtained simultaneously with eROSITA and XMM-Newton during the calibration and performance verification phase of the Spektrum-Roentgen-Gamma mission (SRG). The analysis of the 100 ks deep observation of eROSITA is supported by archival observations of the source, including XMM-Newton, NuSTAR, and NICER. Using XMM-Newton and NICER, we first established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian sigma of about 70 eV that was tentatively identified in a previous long XMM-Newton observation. A second previously discussed absorption feature occurs at 260-265 eV and is described here as an absorption edge. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components that are phenomenologically described here as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power law with photon index Gamma = 2 from the magnetosphere. We created energy-dependent light curves and phase-resolved spectra with a high signal-to-noise ratio. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout similar to 60% of the spin cycle, but it is otherwise undetected. Likewise, its parameters were found to be dependent on phase. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. If the line originates from the stellar surface, it nevertheless likely originates from a different location than the hot polar cap. We also present three families of model atmospheres: a magnetized atmosphere, a condensed surface, and a mixed model. They were applied to the mean observed spectrum, whose continuum fit the observed data well. The atmosphere model, however, predicts distances that are too short. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 10(14) G, which is significantly higher than the field derived from the moderate observed spin-down.
KW  - stars: neutron
KW  - X-rays: stars
KW  - pulsars: individual: PSR B0656+14
Y1  - 2022
U6  - https://doi.org/10.1051/0004-6361/202141105
SN  - 0004-6361
SN  - 1432-0746
VL  - 661
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Ye, Fangyuan
A1  - Zhang, Shuo
A1  - Warby, Jonathan
A1  - Wu, Jiawei
A1  - Gutierrez-Partida, Emilio
A1  - Lang, Felix
A1  - Shah, Sahil
A1  - Saglamkaya, Elifnaz
A1  - Sun, Bowen
A1  - Zu, Fengshuo
A1  - Shoai, Safa
A1  - Wang, Haifeng
A1  - Stiller, Burkhard
A1  - Neher, Dieter
A1  - Zhu, Wei-Hong
A1  - Stolterfoht, Martin
A1  - Wu, Yongzhen
T1  - Overcoming C₆₀-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane
JF  - Nature Communications
N2  - Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C₆₀ interface, limiting the future development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C₆₀ interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23% with a low non-radiative voltage loss of 110 mV, and retain >97% of the initial efficiency after 400 h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells.
Y1  - 2022
U6  - https://doi.org/10.1038/s41467-022-34203-x
SN  - 2041-1723
VL  - 13
IS  - 1
PB  - Springer Nature
CY  - London
ER  - 
TY  - THES
A1  - Schlemm, Tanja
T1  - The marine ice cliff instability of the Antarctic ice sheet
T1  - Die marine Eisklippeninstabilität des antarktischen Eisschildes
BT  - a theory of mélange-buttressed cliff calving and its application in the Parallel Ice Sheet Model
BT  - eine Theorie des Mélange-gebremsten Klippenkalbens und ihre Anwendung im Parallel Ice Sheet Model
N2  - The Antarctic ice sheet is the largest freshwater reservoir worldwide. If it were to melt completely, global sea levels would rise by about 58 m. Calculation of projections of the Antarctic contribution to sea level rise under global warming conditions is an ongoing effort which
yields large ranges in predictions. Among the reasons for this are uncertainties related to the physics of ice sheet modeling. These
uncertainties include two processes that could lead to runaway ice retreat: the Marine Ice Sheet Instability (MISI), which causes rapid grounding line retreat on retrograde bedrock, and the Marine Ice Cliff Instability (MICI), in which tall ice cliffs become unstable and calve off, exposing even taller ice cliffs.
In my thesis, I investigated both marine instabilities (MISI and MICI) using the Parallel Ice Sheet Model (PISM), with a focus on MICI.
N2  - Der antarktische Eisschild ist das größte Süßwasserreservoir der Welt. Würde er vollständig schmelzen, würde der globale Meeresspiegel um etwa 58 m ansteigen. Die Ermittlung von Prognosen über den Beitrag der Antarktis zum Anstieg des Meeresspiegels infolge der globalen Erwärmung ist ein fortlaufender Prozess, der große Unterschiede in den Vorhersagen zur Folge hat. Einer der Gründe dafür sind Ungewissheiten im Zusammenhang mit der Physik der Eisschildmodellierung. Zu diesen Unsicherheiten gehören zwei Prozesse, die zu einem unkontrollierten Eisrückzug führen könnten:
die Marine Ice Sheet Instability (MISI), die zu einem schnellen Rückzug der Grundlinie auf rückläufigem Grundgestein führt, und die Marine Ice Cliff Instability (MICI), bei der hohe Eisklippen instabil werden und abkalben, wodurch noch höhere Eisklippen freigelegt werden.
In meiner Dissertation untersuchte ich beide marinen Instabilitäten (MISI und MICI) mit Hilfe des Parallel Ice Sheet Model (PISM), wobei der Schwerpunkt auf MICI lag.
KW  - Antarctica
KW  - ice sheet modelling
KW  - iceberg calving
KW  - Antarktis
KW  - Eisschildmodellierung
KW  - Eisbergkalbung
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-586333
ER  - 
TY  - THES
A1  - Kuhla, Kilian
T1  - Impact, distribution, and adaptation
T1  - Auswirkung, Verteilung und Anpassung
BT  - how weather extremes threaten the economic network
BT  - wie Wetterextreme das ökonomische Netzwerk bedrohen
N2  - Weather extremes pose a persistent threat to society on multiple layers. Besides an average of ~37,000 deaths per year, climate-related disasters cause destroyed properties and impaired economic activities, eroding people's livelihoods and prosperity. While global temperature rises – caused by anthropogenic greenhouse gas emissions – the direct impacts of climatic extreme events increase and will further intensify without proper adaptation measures. Additionally, weather extremes do not only have local direct effects. Resulting economic repercussions can propagate either upstream or downstream along trade chains causing indirect effects. One approach to analyze these indirect effects within the complex global supply network is the agent-based model Acclimate. Using and extending this loss-propagation model, I focus in this thesis on three aspects of the relation between weather extremes and economic repercussions.

First, extreme weather events cause direct impacts on local economic performance. I compute daily local direct output loss time series of heat stress, river floods, tropical cyclones, and their consecutive occurrence using (near-future) climate projection ensembles. These regional impacts are estimated based on physical drivers and local productivity distribution. Direct effects of the aforementioned disaster categories are widely heterogeneous concerning regional and temporal distribution. As well, their intensity changes differently under future warming. Focusing on the hurricane-impacted capital, I find that long-term growth losses increase with higher heterogeneity of a shock ensemble. 

Second, repercussions are sectorally and regionally distributed via economic ripples within the trading network, causing higher-order effects. I use Acclimate to identify three phases of those economic ripples. Furthermore, I compute indirect impacts and analyze overall regional and global production and consumption changes. Regarding heat stress, global consumer losses double while direct output losses increase by a factor 1.5 between 2000 – 2039. In my research I identify the effect of economic ripple resonance and introduce it to climate impact research. This effect occurs if economic ripples of consecutive disasters overlap, which increases economic responses such as an enhancement of consumption losses. These loss enhancements can even be more amplified with increasing direct output losses, e.g. caused by climate crises.  

Transport disruptions can cause economic repercussions as well. For this, I extend the model Acclimate with a geographical transportation route and expand the decision horizon of economic agents. Using this, I show that policy-induced sudden trade restrictions (e.g. a no-deal Brexit) can significantly reduce the longer-term economic prosperity of affected regions. Analyses of transportation disruptions in typhoon seasons indicate that severely affected regions must reduce production as demand falls during a storm. Substituting suppliers may compensate for fluctuations at the beginning of the storm, which fails for prolonged disruptions.

Third, possible coping mechanisms and adaptation strategies arise from direct and indirect economic responses to weather extremes. Analyzing annual trade changes due to typhoon-induced transport disruptions depict that overall exports rise. This trade resilience increases with higher network node diversification. Further, my research shows that a basic insurance scheme may diminish hurricane-induced long-term growth losses due to faster reconstruction in disasters aftermaths. I find that insurance coverage could be an economically reasonable coping scheme towards higher losses caused by the climate crisis. Indirect effects within the global economic network from weather extremes indicate further adaptation possibilities. For one, diversifying linkages reduce the hazard of sharp price increases. Next to this, close economic interconnections with regions that do not share the same extreme weather season can be economically beneficial in the medium run. Furthermore, economic ripple resonance effects should be considered while computing costs. Overall, an increase in local adaptation measures reduces economic ripples within the trade network and possible losses elsewhere. In conclusion, adaptation measures are necessary and potential present, but it seems rather not possible to avoid all direct or indirect losses.

As I show in this thesis, dynamical modeling gives valuable insights into how direct and indirect economic impacts arise from different categories of weather extremes. Further, it highlights the importance of resolving individual extremes and reflecting amplifying effects caused by incomplete recovery or consecutive disasters.
N2  - Wetterextreme stellen für die Gesellschaft eine anhaltende Bedrohung auf mehreren Ebenen dar. Neben durchschnittlich ~37.000 Todesfällen pro Jahr verursachen meteorologische Katastrophen Eigentumsschäden und Wirtschaftsbeeinträchtigungen, wodurch die Lebensgrundlagen und der Wohlstand der Menschen untergraben werden. Während die globale Temperatur – verursacht durch anthropogene Treibhausgasemissionen – ansteigt, nehmen die direkten Auswirkungen klimatischer Extremereignisse zu und werden sich ohne geeignete Anpassungsmaßnahmen weiter verstärken. 
Hinzu kommt, dass Wetterextreme nicht nur lokal direkte Schäden anrichten, sondern sich wetterbedingte wirtschaftliche Auswirkungen auch entlang der Handelsketten ausbreiten und so indirekte Effekte nach sich ziehen. Ein Ansatz zur Analyse dieser indirekten Auswirkungen innerhalb des komplexen globalen Versorgungsnetzes ist das agentenbasierte Modell Acclimate. 
In meiner Dissertation verwende und erweitere ich dieses Schadenspropagationsmodell, um drei Aspekte der Beziehung zwischen Wetterextremen und wirtschaftlichen Auswirkungen zu untersuchen.

Erstens verursachen extreme Wetterereignisse direkte Schäden in lokaler Wirtschaftsleistung. Die regionalen Auswirkungen werden auf der Grundlage von physikalischen Faktoren und lokalen Produktivitätsverteilungen kalkuliert. Ich berechne tägliche Zeitreihen lokaler Produktionsverluste durch Hitzestress, Überschwemmungen, tropische Wirbelstürme und deren konsekutives Auftreten unter Verwendung von Klimaprojektionsensembles. Die direkten Auswirkungen der oben genannten Katastrophenkategorien sind sehr heterogen in Bezug auf die regionale und zeitliche Verteilung. Ebenso ändert sich ihre Stärke unterschiedlich unter zukünftiger Erwärmung. Meine Forschungsergebnisse zeigen, dass Kapitalstock, welcher von Wirbelstürmen beschädigt ist, langfristige Wachstumsverluste verursacht. Dabei nehmen die Verluste zu, wenn die Heterogenität der Schocks steigt.

Zweitens werden die wetterbedingten Auswirkungen durch wirtschaftliche Wellen innerhalb des Handelsnetzes auf verschiedene Wirtschaftssektoren und Regionen verteilt. In meiner Dissertation, untersuche ich die wirtschaftlichen Wellen mittels Acclimate und mache dabei drei Wellenphasen aus. Darüber hinaus berechne ich indirekte Auswirkungen und analysiere die regionalen und globalen Produktionsveränderungen sowie die Auswirkungen auf Konsumierende. Für letztere verdoppeln sich zwischen 2000 und 2039 die weltweiten Verluste durch Hitzestress, während im selben Zeitraum die direkten Produktionsverluste nur um den Faktor 1.5 steigen. Im Zuge meiner Forschung identifiziere ich den Effekt der ökonomischen Wellenresonanz und führe ihn in die Klimafolgenforschung ein. Dieser Effekt tritt auf, wenn sich die ökonomischen Wellen aufeinanderfolgender Katastrophen überlagern, was wirtschaftliche Reaktionen intensiviert wie beispielsweise eine Steigerung der Konsumverluste. Diese Dynamik der Verluste kann durch zunehmende direkte Produktionsverluste, hervorgerufen etwa durch den Klimawandel, noch verstärkt werden. 

Auch Handelsunterbrechungen können wirtschaftliche Auswirkungen haben. Um diese zu berechnen, erweitere ich das Modell Acclimate um ein geografisches Transportnetzwerk und weite den Entscheidungshorizont der Wirtschaftsakteure aus. Politisch bedingte plötzliche Handelsbeschränkungen (z. B. ein No-Deal-Brexit) können den längerfristigen wirtschaftlichen Wohlstand der betroffenen Regionen erheblich verringern. Analysen von Transportunterbrechungen in der Taifunsaison zeigen, dass stark betroffene Regionen ihre Produktionen reduzieren müssen, wenn die Nachfrage während eines Sturms sinkt. Zu Beginn eines Sturms können Handelsschwankungen durch alternative Lieferanten ausgeglichen werden, was jedoch bei längeren Unterbrechungen nicht mehr gelingt.

Drittens ergeben sich mögliche Anpassungsmechanismen und -strategien aus direkten und indirekten wirtschaftlichen Reaktionen auf Wetterextreme. Die Analyse der jährlichen Handelsveränderungen in der Taifunsaison zeigt, dass Exporte insgesamt zunehmen. Diese Widerstandsfähigkeit des Handels wächst mit einer höheren Diversifizierung der Handelspartner. Weiterhin zeigt meine Forschung an Wirtschaftswachstumsmodellen, dass ein Versicherungssystem langfristige Wachstumsverluste, verursacht durch Tropenstürme, durch schnelleren Wiederaufbau verringern kann. Ich komme zu dem Schluss, dass ein Versicherungsschutz eine wirtschaftlich sinnvolle Anpassungsstrategie gegenüber höheren Schäden durch die Klimakrise sein kann. Ebenso weisen indirekte Auswirkungen von Wetterextremen innerhalb des globalen Wirtschaftsnetzes auf weitere Anpassungsmöglichkeiten hin. Zunächst vermindert eine diversifizierte Vernetzung die Gefahr eines starken Preisanstiegs. Ebenso kann eine enge wirtschaftliche Verflechtung von Regionen, die nicht dieselbe Unwettersaison haben, mittelfristig wirtschaftlich vorteilhaft sein. Weiterhin sollten bei der Berechnung der Kosten wirtschaftliche Resonanzeffekte berücksichtigt werden. Eine Verstärkung der lokalen Anpassungsmaßnahmen verringert die Amplitude ökonomischer Wellen und damit auch potentielle Verluste in anderen Regionen. Insgesamt sind Anpassungsmaßnahmen notwendig, aber es scheint trotz dieser nicht möglich zu sein, alle direkten oder indirekten Verluste zu vermeiden.

Wie ich in meiner Arbeit darlege, gibt die dynamische Modellierung wertvolle Einblicke in die Art und Weise, wie direkte und indirekte wirtschaftliche Auswirkungen durch verschiedene Wetterextreme entstehen. Darüber hinaus wird deutlich, wie wichtig es ist, einzelne Extremereignisse aufzulösen und Verstärkungseffekte zu berücksichtigen, die durch unvollständigen Wiederaufbau oder aufeinanderfolgende Katastrophen verursacht werden.
KW  - climate change
KW  - weather extremes
KW  - macro-economic modelling
KW  - network theory
KW  - economic network
KW  - Klimawandel
KW  - Wetterextreme
KW  - Makroökonomische Modellierung
KW  - Netzwerktheorie
KW  - Ökonomisches Netzwerk
Y1  - 2022
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-552668
ER  - 
TY  - JOUR
A1  - Seroussi, Helene
A1  - Nowicki, Sophie
A1  - Simon, Erika
A1  - Abe-Ouchi, Ayako
A1  - Albrecht, Torsten
A1  - Brondex, Julien
A1  - Cornford, Stephen
A1  - Dumas, Christophe
A1  - Gillet-Chaulet, Fabien
A1  - Goelzer, Heiko
A1  - Golledge, Nicholas R.
A1  - Gregory, Jonathan M.
A1  - Greve, Ralf
A1  - Hoffman, Matthew J.
A1  - Humbert, Angelika
A1  - Huybrechts, Philippe
A1  - Kleiner, Thomas
A1  - Larourl, Eric
A1  - Leguy, Gunter
A1  - Lipscomb, William H.
A1  - Lowry, Daniel
A1  - Mengel, Matthias
A1  - Morlighem, Mathieu
A1  - Pattyn, Frank
A1  - Payne, Anthony J.
A1  - Pollard, David
A1  - Price, Stephen F.
A1  - Quiquet, Aurelien
A1  - Reerink, Thomas J.
A1  - Reese, Ronja
A1  - Rodehacke, Christian B.
A1  - Schlegel, Nicole-Jeanne
A1  - Shepherd, Andrew
A1  - Sun, Sainan
A1  - Sutter, Johannes
A1  - Van Breedam, Jonas
A1  - van de Wal, Roderik S. W.
A1  - Winkelmann, Ricarda
A1  - Zhang, Tong
T1  - initMIP-Antarctica
BT  - an ice sheet model initialization experiment of ISMIP6
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMlP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMlP-Greenland, initMlP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMlP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.
Y1  - 2019
U6  - https://doi.org/10.5194/tc-13-1441-2019
SN  - 1994-0416
SN  - 1994-0424
VL  - 13
IS  - 5
SP  - 1441
EP  - 1471
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Reese, Ronja
A1  - Albrecht, Torsten
A1  - Mengel, Matthias
A1  - Asay-Davis, Xylar
A1  - Winkelmann, Ricarda
T1  - Antarctic sub-shelf melt rates via PICO
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Ocean-induced melting below ice shelves is one of the dominant drivers for mass loss from the Antarctic Ice Sheet at present. An appropriate representation of sub-shelf melt rates is therefore essential for model simulations of marine-based ice sheet evolution. Continental-scale ice sheet models often rely on simple melt-parameterizations, in particular for long-term simulations, when fully coupled ice-ocean interaction becomes computationally too expensive. Such parameterizations can account for the influence of the local depth of the ice-shelf draft or its slope on melting. However, they do not capture the effect of ocean circulation underneath the ice shelf. Here we present the Potsdam Ice-shelf Cavity mOdel (PICO), which simulates the vertical overturning circulation in ice-shelf cavities and thus enables the computation of sub-shelf melt rates consistent with this circulation. PICO is based on an ocean box model that coarsely resolves ice shelf cavities and uses a boundary layer melt formulation. We implement it as a module of the Parallel Ice Sheet Model (PISM) and evaluate its performance under present-day conditions of the Southern Ocean. We identify a set of parameters that yield two-dimensional melt rate fields that qualitatively reproduce the typical pattern of comparably high melting near the grounding line and lower melting or refreezing towards the calving front. PICO captures the wide range of melt rates observed for Antarctic ice shelves, with an average of about 0.1 ma(-1) for cold sub-shelf cavities, for example, underneath Ross or Ronne ice shelves, to 16 ma(-1) for warm cavities such as in the Amundsen Sea region. This makes PICO a computationally feasible and more physical alternative to melt parameterizations purely based on ice draft geometry.
Y1  - 2018
U6  - https://doi.org/10.5194/tc-12-1969-2018
SN  - 1994-0416
SN  - 1994-0424
VL  - 12
IS  - 6
SP  - 1969
EP  - 1985
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Steffen, Will
A1  - Röckstrom, Johan
A1  - Richardson, Katherine
A1  - Lenton, Timothy M.
A1  - Folke, Carl
A1  - Liverman, Diana
A1  - Summerhayes, Colin P.
A1  - Barnosky, Anthony D.
A1  - Cornell, Sarah E.
A1  - Crucifix, Michel
A1  - Donges, Jonathan
A1  - Fetzer, Ingo
A1  - Lade, Steven J.
A1  - Scheffer, Marten
A1  - Winkelmann, Ricarda
A1  - Schellnhuber, Hans Joachim
T1  - Trajectories of the Earth System in the Anthropocene
JF  - Proceedings of the National Academy of Sciences of the United States of America
N2  - We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System-biosphere, climate, and societies-and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.
KW  - Earth System trajectories
KW  - climate change
KW  - Anthropocene
KW  - biosphere feedbacks
KW  - tipping elements
Y1  - 2018
U6  - https://doi.org/10.1073/pnas.1810141115
SN  - 0027-8424
VL  - 115
IS  - 33
SP  - 8252
EP  - 8259
PB  - National Acad. of Sciences
CY  - Washington
ER  - 
TY  - JOUR
A1  - Reese, Ronja
A1  - Winkelmann, Ricarda
A1  - Gudmundsson, Gudmundur Hilmar
T1  - Grounding-line flux formula applied as a flux condition in numerical simulations fails for buttressed Antarctic ice streams
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Currently, several large-scale ice-flow models impose a condition on ice flux across grounding lines using an analytically motivated parameterisation of grounding-line flux. It has been suggested that employing this analytical expression alleviates the need for highly resolved computational domains around grounding lines of marine ice sheets. While the analytical flux formula is expected to be accurate in an unbuttressed flow-line setting, its validity has hitherto not been assessed for complex and realistic geometries such as those of the Antarctic Ice Sheet. Here the accuracy of this analytical flux formula is tested against an optimised ice flow model that uses a highly resolved computational mesh around the Antarctic grounding lines. We find that when applied to the Antarctic Ice Sheet the analytical expression provides inaccurate estimates of ice fluxes for almost all grounding lines. Furthermore, in many instances direct application of the analytical formula gives rise to unphysical complex-valued ice fluxes. We conclude that grounding lines of the Antarctic Ice Sheet are, in general, too highly buttressed for the analytical parameterisation to be of practical value for the calculation of grounding-line fluxes.
Y1  - 2018
U6  - https://doi.org/10.5194/tc-12-3229-2018
SN  - 1994-0416
SN  - 1994-0424
VL  - 12
IS  - 10
SP  - 3229
EP  - 3242
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Ciemer, Catrin
A1  - Rehm, Lars
A1  - Kurths, Jürgen
A1  - Donner, Reik Volker
A1  - Winkelmann, Ricarda
A1  - Boers, Niklas
T1  - An early-warning indicator for Amazon droughts exclusively based on tropical Atlantic sea surface temperatures
JF  - Environmental Research Letters
N2  - Droughts in tropical South America have an imminent and severe impact on the Amazon rainforest and affect the livelihoods of millions of people. Extremely dry conditions in Amazonia have been previously linked to sea surface temperature (SST) anomalies in the adjacent tropical oceans. Although the sources and impacts of such droughts have been widely studied, establishing reliable multi-year lead statistical forecasts of their occurrence is still an ongoing challenge. Here, we further investigate the relationship between SST and rainfall anomalies using a complex network approach. We identify four ocean regions which exhibit the strongest overall SST correlations with central Amazon rainfall, including two particularly prominent regions in the northern and southern tropical Atlantic. Based on the time-dependent correlation between SST anomalies in these two regions alone, we establish a new early-warning method for droughts in the central Amazon basin and demonstrate its robustness in hindcasting past major drought events with lead-times up to 18 months.
KW  - complex networks
KW  - droughts
KW  - prediction
KW  - Amazon rainforest
Y1  - 2019
VL  - 15
IS  - 9
PB  - IOP - Institute of Physics Publishing
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Seroussi, Helene
A1  - Nowicki, Sophie
A1  - Payne, Antony J.
A1  - Goelzer, Heiko
A1  - Lipscomb, William H.
A1  - Abe-Ouchi, Ayako
A1  - Agosta, Cecile
A1  - Albrecht, Torsten
A1  - Asay-Davis, Xylar
A1  - Barthel, Alice
A1  - Calov, Reinhard
A1  - Cullather, Richard
A1  - Dumas, Christophe
A1  - Galton-Fenzi, Benjamin K.
A1  - Gladstone, Rupert
A1  - Golledge, Nicholas R.
A1  - Gregory, Jonathan M.
A1  - Greve, Ralf
A1  - Hattermann, Tore
A1  - Hoffman, Matthew J.
A1  - Humbert, Angelika
A1  - Huybrechts, Philippe
A1  - Jourdain, Nicolas C.
A1  - Kleiner, Thomas
A1  - Larour, Eric
A1  - Leguy, Gunter R.
A1  - Lowry, Daniel P.
A1  - Little, Chistopher M.
A1  - Morlighem, Mathieu
A1  - Pattyn, Frank
A1  - Pelle, Tyler
A1  - Price, Stephen F.
A1  - Quiquet, Aurelien
A1  - Reese, Ronja
A1  - Schlegel, Nicole-Jeanne
A1  - Shepherd, Andrew
A1  - Simon, Erika
A1  - Smith, Robin S.
A1  - Straneo, Fiammetta
A1  - Sun, Sainan
A1  - Trusel, Luke D.
A1  - Van Breedam, Jonas
A1  - van de Wal, Roderik S. W.
A1  - Winkelmann, Ricarda
A1  - Zhao, Chen
A1  - Zhang, Tong
A1  - Zwinger, Thomas
T1  - ISMIP6 Antarctica
BT  - a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and assess the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimates of the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes, forcings employed and initial states of ice sheet models. This study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015-2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between 7:8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between 6 :1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica.
Y1  - 2020
U6  - https://doi.org/10.5194/tc-14-3033-2020
SN  - 1994-0416
SN  - 1994-0424
VL  - 14
IS  - 9
SP  - 3033
EP  - 3070
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Reese, Ronja
A1  - Levermann, Anders
A1  - Albrecht, Torsten
A1  - Seroussi, Helene
A1  - Winkelmann, Ricarda
T1  - The role of history and strength of the oceanic forcing in sea level projections from Antarctica with the Parallel Ice Sheet Model
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Mass loss from the Antarctic Ice Sheet constitutes the largest uncertainty in projections of future sea level rise. Ocean-driven melting underneath the floating ice shelves and subsequent acceleration of the inland ice streams are the major reasons for currently observed mass loss from Antarctica and are expected to become more important in the future. Here we show that for projections of future mass loss from the Antarctic Ice Sheet, it is essential (1) to better constrain the sensitivity of sub-shelf melt rates to ocean warming and (2) to include the historic trajectory of the ice sheet. In particular, we find that while the ice sheet response in simulations using the Parallel Ice Sheet Model is comparable to the median response of models in three Antarctic Ice Sheet Intercomparison projects - initMIP, LARMIP-2 and ISMIP6 - conducted with a range of ice sheet models, the projected 21st century sea level contribution differs significantly depending on these two factors. For the highest emission scenario RCP8.5, this leads to projected ice loss ranging from 1:4 to 4:0 cm of sea level equivalent in simulations in which ISMIP6 ocean forcing drives the PICO ocean box model where parameter tuning leads to a comparably low sub-shelf melt sensitivity and in which no surface forcing is applied. This is opposed to a likely range of 9:1 to 35:8 cm using the exact same initial setup, but emulated from the LARMIP-2 experiments with a higher melt sensitivity, even though both projects use forcing from climate models and melt rates are calibrated with previous oceanographic studies. Furthermore, using two initial states, one with a previous historic simulation from 1850 to 2014 and one starting from a steady state, we show that while differences between the ice sheet configurations in 2015 seem marginal at first sight, the historic simulation increases the susceptibility of the ice sheet to ocean warming, thereby increasing mass loss from 2015 to 2100 by 5% to 50 %. Hindcasting past ice sheet changes with numerical models would thus provide valuable tools to better constrain projections. Our results emphasize that the uncertainty that arises from the forcing is of the same order of magnitude as the ice dynamic response for future sea level projections.
Y1  - 2020
U6  - https://doi.org/10.5194/tc-14-3097-2020
SN  - 1994-0416
SN  - 1994-0424
VL  - 14
IS  - 9
SP  - 3097
EP  - 3110
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Zeitz, Maria
A1  - Levermann, Anders
A1  - Winkelmann, Ricarda
T1  - Sensitivity of ice loss to uncertainty in flow law parameters in an idealized one-dimensional geometry
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Acceleration of the flow of ice drives mass losses in both the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow, melt, and calving. While major advancements have been made by the ice-sheet modeling community in addressing several of the related uncertainties, the flow law, which is at the center of most process-based ice-sheet models, is not in the focus of the current scientific debate. However, recent studies show that the flow law parameters are highly uncertain and might be different from the widely accepted standard values. Here, we use an idealized flow-line setup to investigate how these uncertainties in the flow law translate into uncertainties in flow-driven mass loss. In order to disentangle the effect of future warming on the ice flow from other effects, we perform a suite of experiments with the Parallel Ice Sheet Model (PISM), deliberately excluding changes in the surface mass balance. We find that changes in the flow parameters within the observed range can lead up to a doubling of the flow-driven mass loss within the first centuries of warming, compared to standard parameters. The spread of ice loss due to the uncertainty in flow parameters is on the same order of magnitude as the increase in mass loss due to surface warming. While this study focuses on an idealized flow-line geometry, it is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica.
Y1  - 2020
U6  - https://doi.org/10.5194/tc-14-3537-2020
SN  - 1994-0416
SN  - 1994-0424
VL  - 14
IS  - 10
SP  - 3537
EP  - 3550
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Reese, Ronja
A1  - Gudmundsson, Gudmundur Hilmar
A1  - Levermann, Anders
A1  - Winkelmann, Ricarda
T1  - The far reach of ice-shelf thinning in Antarctica
JF  - Nature climate change
N2  - Floating ice shelves, which fringe most of Antarctica’s coastline, regulate ice flow into the Southern Ocean1,2,3. Their thinning4,5,6,7 or disintegration8,9 can cause upstream acceleration of grounded ice and raise global sea levels. So far the effect has not been quantified in a comprehensive and spatially explicit manner. Here, using a finite-element model, we diagnose the immediate, continent-wide flux response to different spatial patterns of ice-shelf mass loss. We show that highly localized ice-shelf thinning can reach across the entire shelf and accelerate ice flow in regions far from the initial perturbation. As an example, this ‘tele-buttressing’ enhances outflow from Bindschadler Ice Stream in response to thinning near Ross Island more than 900 km away. We further find that the integrated flux response across all grounding lines is highly dependent on the location of imposed changes: the strongest response is caused not only near ice streams and ice rises, but also by thinning, for instance, well-within the Filchner–Ronne and Ross Ice Shelves. The most critical regions in all major ice shelves are often located in regions easily accessible to the intrusion of warm ocean waters10,11,12, stressing Antarctica’s vulnerability to changes in its surrounding ocean.
Y1  - 2017
U6  - https://doi.org/10.1038/s41558-017-0020-x
SN  - 1758-678X
SN  - 1758-6798
VL  - 8
IS  - 1
SP  - 53
EP  - 57
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Albrecht, Torsten
A1  - Winkelmann, Ricarda
A1  - Levermann, Anders
T1  - Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM)
BT  - part 2: parameter ensemble analysis
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - The Parallel Ice Sheet Model (PISM) is applied to the Antarctic Ice Sheet over the last two glacial cycles (approximate to 210 000 years) with a resolution of 16 km. An ensemble of 256 model runs is analyzed in which four relevant model parameters have been systematically varied using full-factorial parameter sampling. Parameters and plausible parameter ranges have been identified in a companion paper (Albrecht et al., 2020) and are associated with ice dynamics, climatic forcing, basal sliding and bed deformation and represent distinct classes of model uncertainties. The model is scored against both modern and geologic data, including reconstructed grounding-line locations, elevation-age data, ice thickness, surface velocities and uplift rates. An aggregated score is computed for each ensemble member that measures the overall model-data misfit, including measurement uncertainty in terms of a Gaussian error model (Briggs and Tarasov, 2013). The statistical method used to analyze the ensemble simulation results follows closely the simple averaging method described in Pollard et al. (2016). 
This analysis reveals clusters of best-fit parameter combinations, and hence a likely range of relevant model and boundary parameters, rather than individual best-fit parameters. The ensemble of reconstructed histories of Antarctic Ice Sheet volumes provides a score-weighted likely range of sea-level contributions since the Last Glacial Maximum (LGM) of 9.4 +/- 4.1m (or 6.5 +/- 2.0 x 10(6) km(3)), which is at the upper range of most previous studies. The last deglaciation occurs in all ensemble simulations after around 12 000 years before present and hence after the meltwater pulse 1A (MWP1a). Our ensemble analysis also provides an estimate of parametric uncertainty bounds for the present-day state that can be used for PISM projections of future sea-level contributions from the Antarctic Ice Sheet.
Y1  - 2020
U6  - https://doi.org/10.5194/tc-14-633-2020
SN  - 1994-0416
SN  - 1994-0424
VL  - 14
IS  - 2
SP  - 633
EP  - 656
PB  - Copernicus Publ.
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Garbe, Julius
A1  - Albrecht, Torsten
A1  - Levermann, Anders
A1  - Donges, Jonathan
A1  - Winkelmann, Ricarda
T1  - The hysteresis of the Antarctic Ice Sheet
JF  - Nature : the international weekly journal of science
N2  - More than half of Earth's freshwater resources are held by the Antarctic Ice Sheet, which thus represents by far the largest potential source for global sea-level rise under future warming conditions(1). Its long-term stability determines the fate of our coastal cities and cultural heritage. Feedbacks between ice, atmosphere, ocean, and the solid Earth give rise to potential nonlinearities in its response to temperature changes. So far, we are lacking a comprehensive stability analysis of the Antarctic Ice Sheet for different amounts of global warming. Here we show that the Antarctic Ice Sheet exhibits a multitude of temperature thresholds beyond which ice loss is irreversible. Consistent with palaeodata(2)we find, using the Parallel Ice Sheet Model(3-5), that at global warming levels around 2 degrees Celsius above pre-industrial levels, West Antarctica is committed to long-term partial collapse owing to the marine ice-sheet instability. Between 6 and 9 degrees of warming above pre-industrial levels, the loss of more than 70 per cent of the present-day ice volume is triggered, mainly caused by the surface elevation feedback. At more than 10 degrees of warming above pre-industrial levels, Antarctica is committed to become virtually ice-free. The ice sheet's temperature sensitivity is 1.3 metres of sea-level equivalent per degree of warming up to 2 degrees above pre-industrial levels, almost doubling to 2.4 metres per degree of warming between 2 and 6 degrees and increasing to about 10 metres per degree of warming between 6 and 9 degrees. Each of these thresholds gives rise to hysteresis behaviour: that is, the currently observed ice-sheet configuration is not regained even if temperatures are reversed to present-day levels. In particular, the West Antarctic Ice Sheet does not regrow to its modern extent until temperatures are at least one degree Celsius lower than pre-industrial levels. Our results show that if the Paris Agreement is not met, Antarctica's long-term sea-level contribution will dramatically increase and exceed that of all other sources. <br /> Modelling shows that the Antarctic Ice Sheet exhibits multiple temperature thresholds beyond which ice loss would become irreversible, and once melted, the ice sheet can regain its previous mass only if the climate cools well below pre-industrial temperatures.
Y1  - 2020
U6  - https://doi.org/10.1038/s41586-020-2727-5
SN  - 0028-0836
SN  - 1476-4687
VL  - 585
IS  - 7826
SP  - 538
EP  - 544
PB  - Macmillan Publishers Limited
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Schlemm, Tanja
A1  - Feldmann, Johannes
A1  - Winkelmann, Ricarda
A1  - Levermann, Anders
T1  - Stabilizing effect of melange buttressing on the marine ice-cliff instability of the West Antarctic Ice Sheet
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Owing to global warming and particularly high regional ocean warming, both Thwaites and Pine Island Glaciers in the Amundsen region of the Antarctic Ice Sheet could lose their buttressing ice shelves over time. We analyse the possible consequences using the parallel ice sheet model (PISM), applying a simple cliff-calving parameterization and an ice melange-buttressing model. We find that the instantaneous loss of ice-shelf buttressing, due to enforced ice-shelf melting, initiates grounding-line retreat and triggers marine ice sheet instability (MISI). As a consequence, the grounding line progresses into the interior of the West Antarctic Ice Sheet and leads to a sea level contribution of 0.6 m within 100 a. By subjecting the exposed ice cliffs to cliff calving using our simplified parameterization, we also analyse marine ice cliff instability (MICI). In our simulations it can double or even triple the sea level contribution depending on the only loosely constrained parameter that determines the maximum cliff-calving rate. The speed of MICI depends on this upper bound of the calving rate, which is given by the ice melange buttressing the glacier. However, stabilization of MICI may occur for geometric reasons. Because the embayment geometry changes as MICI advances into the interior of the ice sheet, the upper bound on calving rates is reduced and the progress of MICI is slowed down. Although we cannot claim that our simulations bear relevant quantitative estimates of the effect of ice-melange buttressing on MICI, the mechanism has the potential to stop the instability. Further research is needed to evaluate its role for the past and future evolution of the Antarctic Ice Sheet.
Y1  - 2022
U6  - https://doi.org/10.5194/tc-16-1979-2022
SN  - 1994-0416
SN  - 1994-0424
VL  - 16
IS  - 5
SP  - 1979
EP  - 1996
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - THES
A1  - Bastian, Martin
T1  - An emergent machine learning approach for seasonal cyclone activity forecasts
N2  - Seasonal forecasts are of great interest in many areas. Knowing the amount of precipitation for the upcoming season in regions of water scarcity would facilitate a better water management. If farmers knew the weather conditions of the upcoming summer at sowing time, they could select those cereal species that are best adapted to these conditions. This would allow farmers to improve the harvest and potentially even reduce the amount of pesticides used. However, the undoubted advantages of seasonal forecasts are often opposed by their high degree of uncertainty. The great challenge of generating seasonal forecasts with lead times of several months mainly originates from the chaotic nature of the earth system. In a chaotic system, even tiny differences in the initial conditions can lead to strong deviations in the system’s state in the long run.

In this dissertation we propose an emergent machine learning approach for seasonal forecasting, called the AnlgModel. The AnlgModel combines the analogue method with myopic feature selection and bootstrapping. To benchmark the abilities of the AnlgModel we apply it to seasonal cyclone activity forecasts in the North Atlantic and Northwest Pacific. The AnlgModel demonstrates competitive hindcast skills with two operational forecasts and even outperforms these for long lead times.

In the second chapter we comprehend the forecasting strategy of the Anlg-Model. We thereby analyse the analogue selection process for the 2017 North Atlantic and the 2018 Northwest Pacific seasonal cyclone activity. The analysis shows that those climate indices which are known to influence the seasonal cyclone activity, such as the Niño 3.4 SST, are correctly represented among the selected analogues. Furthermore the selected analogues reflect large-scale climate patterns that were identified by expert reports as being determinative for these particular seasons.

In the third chapter we analyse the features that are used by the AnlgModel for its predictions. We therefore inspect the feature relevance (FR). The FR patterns learned by the AnlgModel show a high congruence with the predictor regions used by the operational forecasts. However, the AnlgModel also discovered new features, such as the SST anomaly in the Gulf of Guinea during November. This SST pattern exhibits a remarkably high predictive potential for the upcoming Atlantic hurricane activity.

In the final chapter we investigate potential mechanisms, that link two of these regions with high feature relevance to the Atlantic hurricane activity. We mainly focus on ocean surface transport. The ocean surface flow paths are calculated using Lagrangian particle analysis. We demonstrate that the FR patterns in the region of the Canary islands do not correspond with ocean surface transport. It is instead likely that these FR patterns fingerprint a wind transport of latent heat. The second region to be studied is situated in the Gulf of Guinea. Our analysis shows that the FR patterns seen there do fingerprint ocean surface transport. However, our simulations also show that at least one other mechanism is involved in linking the Gulf of Guinea SST anomaly in November to the hurricane activity of the upcoming season.

In this work the AnlgModel does not only demonstrate its outstanding forecast skills but also shows its capabilities as research tool for detecting oceanic and atmospheric mechanisms.
KW  - seasonal cyclone activity forecasts
Y1  - 2023
ER  - 
TY  - JOUR
A1  - Feldmann, Johannes
A1  - Reese, Ronja
A1  - Winkelmann, Ricarda
A1  - Levermann, Anders
T1  - Shear-margin melting causes stronger transient ice discharge than ice-stream melting in idealized simulations
JF  - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union
N2  - Basal ice-shelf melting is the key driver of Antarctica's increasing sea-level contribution. In diminishing the buttressing force of the ice shelves that fringe the ice sheet, the melting increases the ice discharge into the ocean.
Here we contrast the influence of basal melting in two different ice-shelf regions on the time-dependent response of an isothermal, inherently buttressed ice-sheet-shelf system. In the idealized numerical simulations, the basal-melt perturbations are applied close to the grounding line in the ice-shelf's (1) ice-stream region, where the ice shelf is fed by the fastest ice masses that stream through the upstream bed trough and (2) shear margins, where the ice flow is slower.
The results show that melting below one or both of the shear margins can cause a decadal to centennial increase in ice discharge that is more than twice as large compared to a similar perturbation in the ice-stream region. We attribute this to the fact that melt-induced ice-shelf thinning in the central grounding-line region is attenuated very effectively by the fast flow of the central ice stream. In contrast, the much slower ice dynamics in the lateral shear margins of the ice shelf facilitate sustained ice-shelf thinning and thereby foster buttressing reduction.
Regardless of the melt location, a higher melt concentration toward the grounding line generally goes along with a stronger response. Our results highlight the vulnerability of outlet glaciers to basal melting in stagnant, buttressing-relevant ice-shelf regions, a mechanism that may gain importance under future global warming.
Y1  - 2022
U6  - https://doi.org/10.5194/tc-16-1927-2022
SN  - 1994-0416
SN  - 1994-0424
VL  - 16
IS  - 5
SP  - 1927
EP  - 1940
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Wunderling, Nico
A1  - Willeit, Matteo
A1  - Donges, Jonathan
A1  - Winkelmann, Ricarda
T1  - Global warming due to loss of large ice masses and Arctic summer sea ice
JF  - Nature Communications
N2  - Several large-scale cryosphere elements such as the Arctic summer sea ice, the mountain glaciers, the Greenland and West Antarctic Ice Sheet have changed substantially during the last century due to anthropogenic global warming. However, the impacts of their possible future disintegration on global mean temperature (GMT) and climate feedbacks have not yet been comprehensively evaluated. Here, we quantify this response using an Earth system model of intermediate complexity. Overall, we find a median additional global warming of 0.43 degrees C (interquartile range: 0.39-0.46 degrees C) at a CO2 concentration of 400 ppm. Most of this response (55%) is caused by albedo changes, but lapse rate together with water vapour (30%) and cloud feedbacks (15%) also contribute significantly. While a decay of the ice sheets would occur on centennial to millennial time scales, the Arctic might become ice-free during summer within the 21st century. Our findings imply an additional increase of the GMT on intermediate to long time scales. The disintegration of cryosphere elements such as the Arctic summer sea ice, mountain glaciers, Greenland and West Antarctica is associated with temperature and radiative feedbacks. In this work, the authors quantify these feedbacks and find an additional global warming of 0.43 degrees C.
Y1  - 2020
U6  - https://doi.org/10.1038/s41467-020-18934-3
SN  - 2041-1723
VL  - 11
IS  - 1
PB  - Nature Publishing Group
CY  - Berlin
ER  - 
TY  - JOUR
A1  - Klose, Ann Kristin
A1  - Wunderling, Nico
A1  - Winkelmann, Ricarda
A1  - Donges, Jonathan
T1  - What do we mean, 'tipping cascade'?
JF  - Environmental research letters : ERL
N2  - Based on suggested interactions of potential tipping elements in the Earth's climate and in ecological systems, tipping cascades as possible dynamics are increasingly discussed and studied. The activation of such tipping cascades would impose a considerable risk for human societies and biosphere integrity. However, there are ambiguities in the description of tipping cascades within the literature so far. Here we illustrate how different patterns of multiple tipping dynamics emerge from a very simple coupling of two previously studied idealized tipping elements. In particular, we distinguish between a two phase cascade, a domino cascade and a joint cascade. A mitigation of an unfolding two phase cascade may be possible and common early warning indicators are sensitive to upcoming critical transitions to a certain degree. In contrast, a domino cascade may hardly be stopped once initiated and critical slowing down-based indicators fail to indicate tipping of the following element. These different potentials for intervention and anticipation across the distinct patterns of multiple tipping dynamics should be seen as a call to be more precise in future analyses of cascading dynamics arising from tipping element interactions in the Earth system.
KW  - tipping cascade
KW  - domino effect
KW  - tipping interactions
KW  - cascading regime
KW  - shifts
KW  - early warning indicators
Y1  - 2021
U6  - https://doi.org/10.1088/1748-9326/ac3955
SN  - 1748-9326
VL  - 16
IS  - 12
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Kunert, Nina
A1  - Pang, Peter T. H.
A1  - Tews, Ingo
A1  - Coughlin, Michael W.
A1  - Dietrich, Tim
T1  - Quantifying modeling uncertainties when combining multiple gravitational-wave detections from binary neutron star sources
JF  - Physical review D
N2  - With the increasing sensitivity of gravitational-wave detectors, we expect to observe multiple binary neutron-star systems through gravitational waves in the near future. The combined analysis of these gravitational-wave signals offers the possibility to constrain the neutron-star radius and the equation of state of dense nuclear matter with unprecedented accuracy. However, it is crucial to ensure that uncertainties inherent in the gravitational-wave models will not lead to systematic biases when information from multiple detections is combined. To quantify waveform systematics, we perform an extensive simulation campaign of binary neutron-star sources and analyze them with a set of four different waveform models. For our analysis with 38 simulations, we find that statistical uncertainties in the neutron-star radius decrease to 1250 m (2% at 90% credible interval) but that systematic differences between currently employed waveform models can be twice as large. Hence, it will be essential to ensure that systematic biases will not become dominant in inferences of the neutron-star equation of state when capitalizing on future developments.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevD.105.L061301
SN  - 2470-0010
SN  - 2470-0029
VL  - 105
IS  - 6
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Bulla, Mattia
A1  - Coughlin, Michael W.
A1  - Dhawan, Suhail
A1  - Dietrich, Tim
T1  - Multi-messenger constraints on the Hubble constant through combination of gravitational waves, gamma-ray bursts and kilonovae from neutron star mergers
JF  - Universe : open access journal
N2  - The simultaneous detection of gravitational waves and light from the binary neutron star merger GW170817 led to independent measurements of distance and redshift, providing a direct estimate of the Hubble constant H-0 that does not rely on a cosmic distance ladder, nor assumes a specific cosmological model. 
By using gravitational waves as "standard sirens", this approach holds promise to arbitrate the existing tension between the H-0 value inferred from the cosmic microwave background and those obtained from local measurements.
However, the known degeneracy in the gravitational-wave analysis between distance and inclination of the source led to a H-0 value from GW170817 that was not precise enough to resolve the existing tension. 
In this review, we summarize recent works exploiting the viewing-angle dependence of the electromagnetic signal, namely the associated short gamma-ray burst and kilonova, to constrain the system inclination and improve on H-0. 
We outline the key ingredients of the different methods, summarize the results obtained in the aftermath of GW170817 and discuss the possible systematics introduced by each of these methods.
KW  - gravitational waves
KW  - stars: neutron
KW  - stars: binaries
KW  - cosmology: cosmological parameters
KW  - cosmology: distance scale
KW  - cosmology: cosmic background radiation
Y1  - 2022
U6  - https://doi.org/10.3390/universe8050289
SN  - 2218-1997
VL  - 8
IS  - 5
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Ni, Binbin
A1  - Cao, Xing
A1  - Shprits, Yuri
A1  - Summers, Danny
A1  - Gu, Xudong
A1  - Fu, Song
A1  - Lou, Yuequn
T1  - Hot Plasma Effects on the Cyclotron-Resonant Pitch-Angle Scattering Rates of Radiation Belt Electrons Due to EMIC Waves
JF  - Geophysical research letters
N2  - To investigate the hot plasma effects on the cyclotron-resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, calculations of the wave-induced bounce-averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra-relativistic radiation belt electrons by EMIC waves.
Y1  - 2018
U6  - https://doi.org/10.1002/2017GL076028
SN  - 0094-8276
SN  - 1944-8007
VL  - 45
IS  - 1
SP  - 21
EP  - 30
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Bernardi, Rafael L.
A1  - Berdja, Amokrane
A1  - Dani Guzman, Christian
A1  - Torres-Torriti, Miguel
A1  - Roth, Martin M.
T1  - Restoration of images with a spatially varying PSF of the T80-S telescope optical model using neural networks
JF  - Monthly notices of the Royal Astronomical Society
N2  - Most image restoration methods in astronomy rely upon probabilistic tools that infer the best solution for a deconvolution problem. They achieve good performances when the point spread function (PSF) is spatially invariant in the image plane. 

However, this condition is not always satisfied in real optical systems. We propose a new method for the restoration of images affected by static and anisotropic aberrations using Deep Neural Networks that can be directly applied to sky images. 
The network is trained using simulated sky images corresponding to the T80-S Telescope optical model, a 80-cm survey imager at Cerro Tololo (Chile), which are synthesized using a Zernike polynomial representation of the optical system. 

Once trained, the network can be used directly on sky images, outputting a corrected version of the image that has a constant and known PSF across its field of view. The method is to be tested on the T80-S Telescope. 
We present the method and results on synthetic data.
KW  - methods: statistical
KW  - techniques: image processing
Y1  - 2021
U6  - https://doi.org/10.1093/mnras/stab3400
SN  - 0035-8711
SN  - 1365-2966
VL  - 510
IS  - 3
SP  - 4284
EP  - 4294
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Guo, Yingjie
A1  - Ni, Binbin
A1  - Fu, Song
A1  - Wang, Dedong
A1  - Shprits, Yuri
A1  - Zhelavskaya, Irina
A1  - Feng, Minghang
A1  - Guo, Deyu
T1  - Identification of controlling geomagnetic and solar wind factors for magnetospheric chorus intensity using feature selection techniques
JF  - Journal of geophysical research : A, Space physics
N2  - Using over-5-year EMFISIS wave measurements from Van Allen Probes, we present a detailed survey to identify the controlling factors among the geomagnetic indices and solar wind parameters for the 1-min root mean square amplitudes of lower band chorus (LBC) and upper band chorus (UBC). 
A set of important features are automatically determined by feature selection techniques, namely, Random Forest and Maximum Relevancy Minimum Redundancy. Our analysis results indicate the AE index with zero-time-delay dominates the intensity evolution of LBC and UBC, consistent with the evidence that chorus waves prefer to occur and amplify during enhanced substorm periods. Regarding solar wind parameters, solar wind speed and IMF B-z are identified as the controlling factors for chorus wave intensity. Using the combination of all these important features, a predictive neural network model of chorus wave intensity is established to reconstruct the temporal variations of chorus wave intensity, for which application of Random Forest produces the overall best performance. 

Plain Language Summary
Whistler mode chorus waves are electromagnetic waves observed in the low-density region near the geomagnetic equator outside the plasmapause. The dynamics of Earth's radiation belts are largely influenced by chorus waves owing to their dual contributions to both radiation belt electron acceleration and loss. In this study, we use feature selection techniques to identify the controlling geomagnetic and solar wind factors for magnetospheric chorus waves. Feature selection techniques implement the processes which can select the features most influential to the output. 
In this study, the inputs are geomagnetic indices and solar wind parameters and the output is the chorus wave intensity. The results indicate that AE index with zerotime delay dominates the chorus wave intensity. Furthermore, solar wind speed and IMF B-z are identified as the most important solar wind drivers for chorus wave intensity. 
On basis of the combination of all these important geomagnetic and solar wind controlling factors, we develop a neural network model of chorus wave intensity, and find that the model with the inputs identified using the Random Forest method produces the overall best performance.
Y1  - 2021
U6  - https://doi.org/10.1029/2021JA029926
SN  - 2169-9380
SN  - 2169-9402
VL  - 127
IS  - 1
PB  - Wiley
CY  - Hoboken, NJ
ER  - 
TY  - THES
A1  - Smirnov, Artem
T1  - Understanding the dynamics of the near-earth space environment utilizing long-term satellite observations
T1  - Verständnis der Dynamik der erdnahen Weltraumumgebung mit Hilfe von Langzeit-Satellitenbeobachtungen
N2  - The near-Earth space environment is a highly complex system comprised of several regions and particle populations hazardous to satellite operations. The trapped particles in the radiation belts and ring current can cause significant damage to satellites during space weather events, due to deep dielectric and surface charging. Closer to Earth is another important region, the ionosphere, which delays the propagation of radio signals and can adversely affect navigation and positioning. In response to fluctuations in solar and geomagnetic activity, both the inner-magnetospheric and ionospheric populations can undergo drastic and sudden changes within minutes to hours, which creates a challenge for predicting their behavior. Given the increasing reliance of our society on satellite technology, improving our understanding and modeling of these populations is a matter of paramount importance.

In recent years, numerous spacecraft have been launched to study the dynamics of particle populations in the near-Earth space, transforming it into a data-rich environment. To extract valuable insights from the abundance of available observations, it is crucial to employ advanced modeling techniques, and machine learning methods are among the most powerful approaches available. This dissertation employs long-term satellite observations to analyze the processes that drive particle dynamics, and builds interdisciplinary links between space physics and machine learning by developing new state-of-the-art models of the inner-magnetospheric and ionospheric particle dynamics.

The first aim of this thesis is to investigate the behavior of electrons in Earth's radiation belts and ring current. Using ~18 years of electron flux observations from the Global Positioning System (GPS), we developed the first machine learning model of hundreds-of-keV electron flux at Medium Earth Orbit (MEO) that is driven solely by solar wind and geomagnetic indices and does not require auxiliary flux measurements as inputs. We then proceeded to analyze the directional distributions of electrons, and for the first time, used Fourier sine series to fit electron pitch angle distributions (PADs) in Earth's inner magnetosphere. We performed a superposed epoch analysis of 129 geomagnetic storms during the Van Allen Probes era and demonstrated that electron PADs have a strong energy-dependent response to geomagnetic activity. Additionally, we showed that the solar wind dynamic pressure could be used as a good predictor of the PAD dynamics. Using the observed dependencies, we created the first PAD model with a continuous dependence on L, magnetic local time (MLT) and activity, and developed two techniques to reconstruct near-equatorial electron flux observations from low-PA data using this model.

The second objective of this thesis is to develop a novel model of the topside ionosphere. To achieve this goal, we collected observations from five of the most widely used ionospheric missions and intercalibrated these data sets. This allowed us to use these data jointly for model development, validation, and comparison with other existing empirical models. We demonstrated, for the first time, that ion density observations by Swarm Langmuir Probes exhibit overestimation (up to ~40-50%) at low and mid-latitudes on the night side, and suggested that the influence of light ions could be a potential cause of this overestimation. To develop the topside model, we used 19 years of radio occultation (RO) electron density profiles, which were fitted with a Chapman function with a linear dependence of scale height on altitude. This approximation yields 4 parameters, namely the peak density and height of the F2-layer and the slope and intercept of the linear scale height trend, which were modeled using feedforward neural networks (NNs). The model was extensively validated against both RO and in-situ observations and was found to outperform the International Reference Ionosphere (IRI) model by up to an order of magnitude. Our analysis showed that the most substantial deviations of the IRI model from the data occur at altitudes of 100-200 km above the F2-layer peak. The developed NN-based ionospheric model reproduces the effects of various physical mechanisms observed in the topside ionosphere and provides highly accurate electron density predictions.

This dissertation provides an extensive study of geospace dynamics, and the main results of this work contribute to the improvement of models of plasma populations in the near-Earth space environment.
N2  - Die erdnahe Weltraumumgebung ist ein hochkomplexes System, das aus mehreren Regionen und Partikelpopulationen besteht, die für den Satellitenbetrieb gefährlich sind. Die in den Strahlungsgürteln und dem Ringstrom gefangenen Teilchen können bei Weltraumwetterereignissen aufgrund der tiefen dielektrischen und oberflächlichen Aufladung erhebliche Schäden an Satelliten verursachen. Näher an der Erde liegt eine weitere wichtige Region, die Ionosphäre, die die Ausbreitung von Funksignalen verzögert und die Navigation und Positionsbestimmung beeinträchtigen kann. Als Reaktion auf Fluktuationen der solaren und geomagnetischen Aktivität können sowohl die Populationen der inneren Magnetosphäre als auch der Ionosphäre innerhalb von Minuten bis Stunden drastische und plötzliche Veränderungen erfahren, was eine Herausforderung für die Vorhersage ihres Verhaltens darstellt. Angesichts der zunehmenden Abhängigkeit unserer Gesellschaft von der Satellitentechnologie ist ein besseres Verständnis und eine bessere Modellierung dieser Populationen von größter Bedeutung.

In den letzten Jahren wurden zahlreiche Raumsonden gestartet, um die Dynamik von Partikelpopulationen im erdnahen Weltraum zu untersuchen, was diesen in eine datenreiche Umgebung verwandelt hat. Um aus der Fülle der verfügbaren Beobachtungen wertvolle Erkenntnisse zu gewinnen, ist der Einsatz fortschrittlicher Modellierungstechniken unabdingbar, und Methoden des maschinellen Lernens gehören zu den leistungsfähigsten verfügbaren Ansätzen. Diese Dissertation nutzt langfristige Satellitenbeobachtungen, um die Prozesse zu analysieren, die die Teilchendynamik antreiben, und schafft interdisziplinäre Verbindungen zwischen Weltraumphysik und maschinellem Lernen, indem sie neue hochmoderne Modelle der innermagnetosphärischen und ionosphärischen Teilchendynamik entwickelt.

Das erste Ziel dieser Arbeit ist es, das Verhalten von Elektronen im Strahlungsgürtel und Ringstrom der Erde zu untersuchen. Unter Verwendung von ~18 Jahren Elektronenflussbeobachtungen des Global Positioning System (GPS) haben wir das erste maschinelle Lernmodell des Elektronenflusses im mittleren Erdorbit (MEO) entwickelt, das ausschließlich durch Sonnenwind und geomagnetische Indizes gesteuert wird und keine zusätzlichen Flussmessungen als Eingaben benötigt. Anschließend analysierten wir die Richtungsverteilungen der Elektronen und verwendeten zum ersten Mal Fourier-Sinus-Reihen, um die Elektronen-Stellwinkelverteilungen (PADs) in der inneren Magnetosphäre der Erde zu bestimmen. Wir führten eine epochenübergreifende Analyse von 129 geomagnetischen Stürmen während der Van-Allen-Sonden-Ära durch und zeigten, dass die Elektronen-PADs eine starke energieabhängige Reaktion auf die geomagnetische Aktivität haben. Außerdem konnten wir zeigen, dass der dynamische Druck des Sonnenwindes als guter Prädiktor für die PAD-Dynamik verwendet werden kann. Anhand der beobachteten Abhängigkeiten haben wir das erste PAD-Modell mit einer kontinuierlichen Abhängigkeit von L, der magnetischen Ortszeit (MLT) und der Aktivität erstellt und zwei Techniken entwickelt, um die Beobachtungen des äquatornahen Elektronenflusses aus Daten mit niedrigem Luftdruck mit Hilfe dieses Modells zu rekonstruieren.

Das zweite Ziel dieser Arbeit ist die Entwicklung eines neuen Modells der Topside-Ionosphäre. Um dieses Ziel zu erreichen, haben wir Beobachtungen von fünf der meistgenutzten Ionosphärenmissionen gesammelt und diese Datensätze interkalibriert. So konnten wir diese Daten gemeinsam für die Modellentwicklung, die Validierung und den Vergleich mit anderen bestehenden empirischen Modellen nutzen. Wir haben zum ersten Mal gezeigt, dass die Ionendichtebeobachtungen von Swarm-Langmuir-Sonden in niedrigen und mittleren Breiten auf der Nachtseite eine Überschätzung (bis zu ~40-50%) aufweisen, und haben vorgeschlagen, dass der Einfluss leichter Ionen eine mögliche Ursache für diese Überschätzung sein könnte. Zur Entwicklung des Oberseitenmodells wurden 19 Jahre lang Elektronendichteprofile aus der Radio-Okkultation (RO) verwendet, die mit einer Chapman-Funktion mit einer linearen Abhängigkeit der Skalenhöhe von der Höhe angepasst wurden. Aus dieser Näherung ergeben sich 4 Parameter, nämlich die Spitzendichte und die Höhe der F2-Schicht sowie die Steigung und der Achsenabschnitt des linearen Trends der Skalenhöhe, die mit Hilfe von neuronalen Feedforward-Netzwerken (NN) modelliert wurden. Das Modell wurde sowohl anhand von RO- als auch von In-situ-Beobachtungen umfassend validiert und übertrifft das Modell der Internationalen Referenz-Ionosphäre (IRI). Unsere Analyse zeigte, dass die größten Abweichungen des IRI-Modells von den Daten in Höhen von 100-200 km über der F2-Schichtspitze auftreten. Das entwickelte NN-basierte Ionosphärenmodell reproduziert die Auswirkungen verschiedener physikalischer Mechanismen, die in der Topside-Ionosphäre beobachtet werden, und liefert sehr genaue Vorhersagen der Elektronendichte.

Diese Dissertation bietet eine umfassende Untersuchung der Dynamik in der Geosphäre, und die wichtigsten Ergebnisse dieser Arbeit tragen zur Verbesserung der Modelle von Plasmapopulationen in der erdnahen Weltraumumgebung bei.
KW  - Ionosphere
KW  - radiation belts
KW  - ring current
KW  - space physics
KW  - empirical modeling
KW  - machine learning
KW  - gradient boosting
KW  - neural networks
KW  - Ionosphäre
KW  - empirische Modellierung
KW  - Gradient Boosting
KW  - maschinelles Lernen
KW  - neuronale Netze
KW  - Strahlungsgürtel
KW  - Ringstrom
KW  - Weltraumphysik
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-613711
ER  - 
TY  - JOUR
A1  - Wang, Dedong
A1  - Shprits, Yuri
A1  - Zhelayskaya, Irina S.
A1  - Agapitov, Oleksiy
A1  - Drozdov, Alexander
A1  - Aseev, Nikita
T1  - Analytical chorus wave model derived from van Allen Probe Observations
JF  - Journal of geophysical research : Space physics
N2  - Chorus waves play an important role in the dynamic evolution of energetic electrons in the Earth's radiation belts and ring current. Using more than 5 years of Van Allen Probe data, we developed a new analytical model for upper‐band chorus (UBC; 0.5fce < f < fce) and lower‐band chorus (LBC; 0.05fce < f < 0.5fce) waves, where fce is the equatorial electron gyrofrequency. By applying polynomial fits to chorus wave root mean square amplitudes, we developed regression models for LBC and UBC as a function of geomagnetic activity (Kp), L, magnetic latitude (λ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ, L, and MLT as Kp‐scaling law to simplify the calculation of diffusion coefficients and inclusion into particle tracing codes. Frequency models for UBC and LBC are also developed, which depends on MLT and magnetic latitude. This empirical model is valid in all MLTs, magnetic latitude up to 20°, Kp ≤ 6, L‐shell range from 3.5 to 6 for LBC and from 4 to 6 for UBC. The dependence of root mean square amplitudes on L are different for different bands, which implies different energy sources for different wave bands. This analytical chorus wave model is convenient for inclusion in quasi‐linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere, especially for the newly developed four‐dimensional codes, which require significantly improved wave parameterizations.
KW  - chorus waves
KW  - radiation belt electrons
KW  - ring current electrons
KW  - analytical model
KW  - wave-particle interactions
KW  - diffusion coefficients
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026183
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 2
SP  - 1063
EP  - 1084
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Smirnov, Artem G.
A1  - Kronberg, Elena A.
A1  - Latallerie, F.
A1  - Daly, Patrick W.
A1  - Aseev, Nikita
A1  - Shprits, Yuri
A1  - Kellerman, Adam C.
A1  - Kasahara, Satoshi
A1  - Turner, Drew L.
A1  - Taylor, M. G. G. T.
T1  - Electron Intensity Measurements by the Cluster/RAPID/IES Instrument in Earth's Radiation Belts and Ring Current
JF  - Space Weather: The International Journal of Research and Applications
N2  - Plain Language Summary Radiation belts of the Earth, which are the zones of charged energetic particles trapped by the geomagnetic field, comprise enormous and dynamic systems. While the inner radiation belt, composed mainly of high-energy protons, is relatively stable, the outer belt, filled with energetic electrons, is highly variable and depends substantially on solar activity. Hence, extended reliable observations and the improved models of the electron intensities in the outer belt depending on solar wind parameters are necessary for prediction of their dynamics. The Cluster mission has been measuring electron flux intensities in the radiation belts since its launch in 2000, thus providing a huge dataset that can be used for radiation belts analysis. Using 16 years of electron measurements by the Cluster mission corrected for background contamination, we derived a uniform linear-logarithmic dependence of electron fluxes in the outer belt on the solar wind dynamic pressure.
Y1  - 2019
U6  - https://doi.org/10.1029/2018SW001989
SN  - 1542-7390
VL  - 17
IS  - 4
SP  - 553
EP  - 566
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Dobynde, M. I.
A1  - Effenberger, Frederic
A1  - Kartashov, D. A.
A1  - Shprits, Yuri
A1  - Shurshakov, V. A.
T1  - Ray-tracing simulation of the radiation dose distribution on the surface of the spherical phantom of the MATROSHKA-R experiment onboard the ISS
JF  - Life sciences in space research
N2  - Space radiation is one of the main concerns for human space flights. The prediction of the radiation dose for the actual spacecraft geometry is very important for the planning of long-duration missions. We present a numerical method for the fast calculation of the radiation dose rate during a space flight. We demonstrate its application for dose calculations during the first and the second sessions of the MATROSHKA-R space experiment with a spherical tissue-equivalent phantom. The main advantage of the method is the short simulation time, so it can be applied for urgent radiation dose calculations for low-Earth orbit space missions. The method uses depth-dose curve and shield-and-composition distribution functions to calculate a radiation dose at the point of interest. The spacecraft geometry is processed into a shield-and-composition distribution function using a ray-tracing method. Depth-dose curves are calculated using the GEANT4 Monte-Carlo code (version 10.00.P02) for a double-layer aluminum-water shielding. Aluminum-water shielding is a good approximation of the real geometry, as water is a good equivalent for biological tissues, and aluminum is the major material of spacecraft bodies.
KW  - Space radiation
KW  - Radiation protection
KW  - Radiation dose calculation
KW  - GEANT4 modeling
KW  - Radiation on the ISS
KW  - MATROSHKA-R
Y1  - 2019
U6  - https://doi.org/10.1016/j.lssr.2019.04.001
SN  - 2214-5524
SN  - 2214-5532
VL  - 21
SP  - 65
EP  - 72
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Zhu, Hui
A1  - Chen, Lunjin
A1  - Liu, Xu
A1  - Shprits, Yuri
T1  - Modulation of locally generated equatorial noise by ULF wave
JF  - Journal of geophysical research : Space physics
N2  - In this paper we report a rare and fortunate event of fast magnetosonic (MS, also called equatorial noise) waves modulated by compressional ultralow frequency (ULF) waves measured by Van Allen Probes. The characteristics of MS waves, ULF waves, proton distribution, and their potential correlations are analyzed. The results show that ULF waves can modulate the energetic ring proton distribution and in turn modulate the MS generation. Furthermore, the variation of MS intensities is attributed to not only ULF wave activities but also the variation of background parameters, for example, number density. The results confirm the opinion that MS waves are generated by proton ring distribution and propose a new modulation phenomenon.
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026199
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 4
SP  - 2779
EP  - 2787
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Shprits, Yuri
A1  - Menietti, J. D.
A1  - Drozdov, Alexander
A1  - Horne, Richard B.
A1  - Woodfield, Emma E.
A1  - Groene, J. B.
A1  - de Soria-Santacruz, M.
A1  - Averkamp, T. F.
A1  - Garrett, H.
A1  - Paranicas, C.
A1  - Gurnett, Don A.
T1  - Strong whistler mode waves observed in the vicinity of Jupiter’s moons
JF  - Nature Communications
N2  - Understanding of wave environments is critical for the understanding of how particles are accelerated and lost in space. This study shows that in the vicinity of Europa and Ganymede, that respectively have induced and internal magnetic fields, chorus wave power is significantly increased. The observed enhancements are persistent and exceed median values of wave activity by up to 6 orders of magnitude for Ganymede. Produced waves may have a pronounced effect on the acceleration and loss of particles in the Jovian magnetosphere and other astrophysical objects. The generated waves are capable of significantly modifying the energetic particle environment, accelerating particles to very high energies, or producing depletions in phase space density. Observations of Jupiter’s magnetosphere provide a unique opportunity to observe how objects with an internal magnetic field can interact with particles trapped in magnetic fields of larger scale objects.
Y1  - 2018
U6  - https://doi.org/10.1038/s41467-018-05431-x
SN  - 2041-1723
VL  - 9
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Shprits, Yuri
A1  - Angelopoulos, V.
A1  - Russell, C. T.
A1  - Strangeway, R. J.
A1  - Runov, A.
A1  - Turner, D.
A1  - Caron, R.
A1  - Cruce, P.
A1  - Leneman, D.
A1  - Michaelis, I.
A1  - Petrov, V.
A1  - Panasyuk, M.
A1  - Yashin, I.
A1  - Drozdov, Alexander
A1  - Russell, C. L.
A1  - Kalegaev, V.
A1  - Nazarkov, I.
A1  - Clemmons, J. H.
T1  - Scientific Objectives of Electron Losses and Fields INvestigation Onboard Lomonosov Satellite
JF  - Space science reviews
N2  - The objective of the Electron Losses and Fields INvestigation on board the Lomonosov satellite ( ELFIN-L) project is to determine the energy spectrum of precipitating energetic electrons and ions and, together with other polar-orbiting and equatorial missions, to better understand the mechanisms responsible for scattering these particles into the atmosphere. This mission will provide detailed measurements of the radiation environment at low altitudes. The 400-500 km sun-synchronous orbit of Lomonosov is ideal for observing electrons and ions precipitating into the atmosphere. This mission provides a unique opportunity to test the instruments. Similar suite of instruments will be flown in the future NSF-and NASA-supported spinning CubeSat ELFIN satellites which will augment current measurements by providing detailed information on pitch-angle distributions of precipitating and trapped particles.
KW  - Magnetospheric physics
KW  - Observations
KW  - Particles precipitating
KW  - Particles trapped
KW  - Radiation belts
Y1  - 2017
U6  - https://doi.org/10.1007/s11214-017-0455-4
SN  - 0038-6308
SN  - 1572-9672
VL  - 214
IS  - 1
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - GEN
A1  - Shprits, Yuri
A1  - Zhelavskaya, Irina
A1  - Green, Janet C.
A1  - Pulkkinen, Antti A.
A1  - Horne, Richard B.
A1  - Pitchford, David
A1  - Glover, Alexi
T1  - Discussions on Stakeholder Requirements for Space Weather-Related Models
T2  - Space Weather: The International Journal of Research and Applications
N2  - Participants of the 2017 European Space Weather Week in Ostend, Belgium, discussed the stakeholder requirements for space weather-related models. It was emphasized that stakeholders show an increased interest in space weather-related models. Participants of the meeting discussed particular prediction indicators that can provide first-order estimates of the impact of space weather on engineering systems.
KW  - 7924
KW  - 7934
KW  - 7959
Y1  - 2018
U6  - https://doi.org/10.1002/2018SW001864
SN  - 1542-7390
VL  - 16
IS  - 4
SP  - 341
EP  - 342
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - GEN
A1  - Shprits, Yuri
A1  - Horne, Richard B.
A1  - Kellerman, Adam C.
A1  - Drozdov, Alexander
T1  - The dynamics of Van Allen belts revisited
T2  - Nature physics
N2  - In an effort to explain the formation of a narrow third radiation belt at ultra-relativistic energies detected during a solar storm in September 20121, Mann et al.2 present simulations from which they conclude it arises from a process of outward radial diffusion alone, without the need for additional loss processes from higher frequency waves. The comparison of observations with the model in Figs 2 and 3 of their Article clearly shows that even with strong radial diffusion rates, the model predicts a third belt near L* = 3 that is twice as wide as observed and approximately an order of magnitude more intense. We therefore disagree with their interpretation that “the agreement between the absolute fluxes from the model and those observed by REPT [the Relativistic Electron Proton Telescope] shown on Figs 2 and 3 is excellent.”

Previous studies3 have shown that outward radial diffusion plays a very important role in the dynamics of the outer belt and is capable of explaining rapid reductions in the electron flux. It has also been shown that it can produce remnant belts (Fig. 2 of a long-term simulation study4). However, radial diffusion alone cannot explain the formation of the narrow third belt at multi-MeV during September 2012. An additional loss mechanism is required.

Higher radial diffusion rates cannot improve the comparison of model presented by Mann et al. with observations. A further increase in the radial diffusion rates (reported in Fig. 4 of the Supplementary Information of ref. 2) results in the overestimation of the outer belt fluxes by up to three orders of magnitude at energy of 3.4 MeV.

Observations at 2 MeV, where belts show only a two-zone structure, were not presented by Mann et al. Moreover, simulations of electrons with energies below 2 MeV with the same diffusion rates and boundary conditions used by the authors would probably produce very strong depletions down to L = 3–3.5, where L is radial distance from the centre of the Earth to the given field line in the equatorial plane. Observations do not show a non-adiabatic loss below L ∼ 4.5 for 2 MeV. Such different dynamics between 2 MeV and above 4 MeV at around L = 3.5 are another indication that particles are scattered by electromagnetic ion cyclotron (EMIC) waves that affect only energies above a certain threshold.

Observations of the phase space density (PSD) provide additional evidence for the local loss of electrons. Around L* = 3.5–4 PSD shows significant decrease by an order of magnitude starting in the afternoon of 3 September (Fig. 1a), while PSD above L* = 4 is increasing. The minimum in PSD between L* = 3.5–4 continues to decrease until 4 September. This evolution demonstrates that the loss is not produced by outward diffusion. Radial diffusion cannot produce deepening minima, as it works to smooth gradients. Just as growing peaks in PSD show the presence of localized acceleration5, deepening minima show the presence of localized loss.
Figure 1: Time evolution of radiation profiles in electron PSD at relativistic and ultra-relativistic energies.
figure 1

a, Similar to Supplementary Fig. 3 of ref. 2, but using TS07D model10 and for μ = 2,500 MeV G−1, K = 0.05 RE G0.5 (where RE is the radius of the Earth). b, Similar to Supplementary Fig. 3 of ref. 2, but using TS07D model and for μ = 700 MeV G−1, corresponding to MeV energies in the heart of the belt. Minimum in PSD in the heart of the multi-MeV electron radiation belt between 3.5 and 4 RE deepening between the afternoon of 3 September and 5 September clearly show that the narrow remnant belt at multi-MeV below 3.5 RE is produced by the local loss.
Full size image

The minimum in the outer boundary is reached on the evening of 2 September. After that, the outer boundary moves up, while the minimum decreases by approximately an order of magnitude, clearly showing that this main decrease cannot be explained by outward diffusion, and requires additional loss processes. The analysis of profiles of PSD is a standard tool used, for example, in the study about electron acceleration5 and routinely used by the entire Van Allen Probes team. In the Supplementary Information, we show that this analysis is validated by using different magnetic field models. The Supplementary Information also shows that measurements are above background noise.

Deepening minima at multi-MeV during the times when the boundary flux increases are clearly seen in Fig. 1a. They show that there must be localized loss, as radial diffusion cannot produce a minimum that becomes lower with time. At lower energies of 1–2 MeV, which corresponds to lower values of the first adiabatic invariant μ (Fig. 1b), the profiles are monotonic between L* = 3–3.5, consistent with the absence of scattering by EMIC waves that affect only electrons above a certain energy threshold6,7,8,9.

In summary, the results of the modelling and observations presented by Mann et al. do not lend support to the claim of explaining the dynamics of the ultra-relativistic third Van Allen radiation belt in terms of an outward radial diffusion process alone. While the outward radial diffusion driven by the loss to the magnetopause2 is certainly operating during this storm, there is compelling observational and modelling2,6 evidence that shows that very efficient localized electron loss operates during this storm at multi-MeV energies, consistent with localized loss produced by EMIC waves.
Y1  - 2018
U6  - https://doi.org/10.1038/nphys4350
SN  - 1745-2473
SN  - 1745-2481
VL  - 14
IS  - 2
SP  - 102
EP  - 103
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Cervantes Villa, Juan Sebastian
A1  - Shprits, Yuri
A1  - Aseev, Nikita
A1  - Allison, Hayley J.
T1  - Quantifying the effects of EMIC wave scattering and magnetopause shadowing in the outer electron radiation belt by means of data assimilation
JF  - Journal of geophysical research : Space physics
N2  - In this study we investigate two distinct loss mechanisms responsible for the rapid dropouts of radiation belt electrons by assimilating data from Van Allen Probes A and B and Geostationary Operational Environmental Satellites (GOES) 13 and 15 into a 3-D diffusion model. In particular, we examine the respective contribution of electromagnetic ion cyclotron (EMIC) wave scattering and magnetopause shadowing for values of the first adiabatic invariant mu ranging from 300 to 3,000 MeV G(-1). We inspect the innovation vector and perform a statistical analysis to quantitatively assess the effect of both processes as a function of various geomagnetic indices, solar wind parameters, and radial distance from the Earth. Our results are in agreement with previous studies that demonstrated the energy dependence of these two mechanisms. We show that EMIC wave scattering tends to dominate loss at lower L shells, and it may amount to between 10%/hr and 30%/hr of the maximum value of phase space density (PSD) over all L shells for fixed first and second adiabatic invariants. On the other hand, magnetopause shadowing is found to deplete electrons across all energies, mostly at higher L shells, resulting in loss from 50%/hr to 70%/hr of the maximum PSD. Nevertheless, during times of enhanced geomagnetic activity, both processes can operate beyond such location and encompass the entire outer radiation belt.
Y1  - 2020
U6  - https://doi.org/10.1029/2020JA028208
SN  - 2169-9380
SN  - 2169-9402
VL  - 125
IS  - 8
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Kim, Kyung-Chan
A1  - Shprits, Yuri
T1  - Survey of the Favorable Conditions for Magnetosonic Wave Excitation
JF  - Journal of geophysical research : Space physics
N2  - The ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe-A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp < f ≤ 0.5 fLHR and 0.5 fLHR < f < fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower-frequency bands (fcp < f ≤ 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low-frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time.
Y1  - 2018
U6  - https://doi.org/10.1002/2017JA024865
SN  - 2169-9380
SN  - 2169-9402
VL  - 123
IS  - 1
SP  - 400
EP  - 413
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Shprits, Yuri
A1  - Kellerman, Adam C .
A1  - Aseev, Nikita
A1  - Drozdov, Alexander
A1  - Michaelis, Ingo
T1  - Multi-MeV electron loss in the heart of the radiation belts
JF  - Geophysical research letters
N2  - Significant progress has been made in recent years in understanding acceleration mechanisms in the Earth's radiation belts. In particular, a number of studies demonstrated the importance of the local acceleration by analyzing the radial profiles of phase space density (PSD) and observing building up peaks in PSD. In this study, we focus on understanding of the local loss using very similar tools. The profiles of PSD for various values of the first adiabatic invariants during the previously studied 17 January 2013 storm are presented and discussed. The profiles of PSD show clear deepening minimums consistent with the scattering by electromagnetic ion cyclotron waves. Long-term evolution shows that local minimums in PSD can persist for relatively long times. During considered interval of time the deepening minimums were observed around L* = 4 during 17 January 2013 storm and around L* = 3.5 during 1 March 2013 storm. This study shows a new method that can help identify the location, magnitude, and time of the local loss and will help quantify local loss in the future. This study also provides additional clear and definitive evidence that local loss plays a major role for the dynamics of the multi-MeV electrons.
Y1  - 2017
U6  - https://doi.org/10.1002/2016GL072258
SN  - 0094-8276
SN  - 1944-8007
VL  - 44
IS  - 3
SP  - 1204
EP  - 1209
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - THES
A1  - Maiti, Snehanshu
T1  - Magnetohydrodynamic turbulence and cosmic ray transport
T1  - Magnetohydrodynamische Turbulenz und Transport kosmischer Strahlung
N2  - The first part of the thesis studies the properties of fast mode in magneto hydro-dynamic (MHD) turbulence. 1D and 3D numerical simulations are carried out to generate decaying fast mode MHD turbulence. The injection of waves are carried out in a collinear and isotropic fashion to generate fast mode turbulence. The properties of fast mode turbulence are analyzed by studying their energy spectral density, 2D structure functions and energy decay/cascade time. The injection wave vector is varied to study the dependence of the above properties on the injection wave vectors. The 1D energy spectrum obtained for the velocity and magnetic fields has 𝐸 (𝑘) ∝ 𝑘−2. The 2D energy spectrum and 2D structure functions in parallel and perpendicular directions shows that fast mode turbulence generated is isotropic in nature. The cascade/decay rate of fast mode MHD turbulence is proportional to 𝑘−0.5 for different kinds of wave vector injection. Simulations are also carried out in 1D and 3D to compare balanced and imbalanced turbulence. The results obtained shows that while 1D imbalanced turbulence decays faster than 1D balanced turbulence, there is no difference in the decay of 3D balanced and imbalanced turbulence for the current resolution of 512 grid points.
"The second part of the thesis studies cosmic ray (CR) transport in driven MHD turbulence and is strongly dependent on it’s properties. Test particle simulations are carried out to study CR interaction with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. The results confirms that the fast modes dominate the CR propagation, whereas Alfvén, slow modes are much less efficient with similar pitch angle scattering rates. The cross field transport on large and small scales are investigated next. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by 𝑀𝜁𝐴 compared to the parallel diffusion coefficients, with 𝜁 closer to 4 in Alfvén modes than that in total turbulence as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale 𝐿, both the local and global magnetic reference frames are adopted. Super diffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfvén modes show clear Richardson diffusion in the local reference frame. The diffusion transition smoothly from the Richardson’s one with index 1.5 to normal diffusion as particle’s mean free path decreases from 𝜆∥ ≫ 𝐿 to 𝜆∥ ≪ 𝐿. These results have broad applications to CRs in various astrophysical environments".
N2  - Der erste Teil der Arbeit untersucht die Eigenschaften des schnellen Modus in magnetohydrodynamischen (MHD) Turbulenzen. Es werden numerische 1D- und 3D-Simulationen durchgeführt, um eine abklingende Fast-Mode-MHD-Turbulenz zu erzeugen. Die Injektion von Wellenvektoren wird kollinear und isotrop durchgeführt, um Fast-Mode-Turbulenzen zu erzeugen. Die Eigenschaften der Fast-Mode-Turbulenz werden durch die Untersuchung ihrer Energie-Spektraldichte, 2D-Strukturfunktionen und Energieabfall-/Kaskadenzeit analysiert. Die Injektionswellenvektoren werden in verschiedenen Simulationen für unterschiedliche Arten der Injektion variiert, um die Abhängigkeit der oben genannten Eigenschaften von den Injektionswellenvektoren zu untersuchen. Das für die Geschwindigkeits- und Magnetfelder erhaltene 1D-Energiespektrum hat E(k) ∝ k−2. Das 2D-Energiespektrum und die 2D-Strukturfunktionen in parallelen und senkrechten Richtungen zeigen, dass die erzeugte Fast-Mode-Turbulenz von Natur aus isotrop ist. Die Kaskaden-/Zerfallsrate der Fast-Mode-MHD-Turbulenz ist proportional zu k−0.5 für verschiedene Arten der Wellenvektorinjektion. Es werden auch Simulationen in 1D und 3D durchgeführt, um ausgeglichene und unausgeglichene Turbulenzen zu vergleichen. Die Ergebnisse zeigen, dass eine unausgewogene 1D-Turbulenz schneller abklingt als eine ausgeglichene 1D-Turbulenz, während es bei der derzeitigen Auflösung von 512 Gitterpunkten keinen Unterschied im Abklingen von ausgeglichener und unausgewogener 3D-Turbulenz gibt.
Der zweite Teil der Arbeit untersucht den Transport kosmischer Strahlung (CR) in angetriebenen MHD-Turbulenzen und ist stark von deren Eigenschaften abhängig. Es werden Testpartikelsimulationen durchgeführt, um die Wechselwirkung von kosmischer Strahlung sowohl mit der gesamten MHD-Turbulenz als auch mit zerlegten MHD-Moden zu untersuchen. Aus den Flugbahnen der Testteilchen in der Turbulenz werden die räumlichen Diffusionskoeffizienten und die Diffusionskoeffizienten für die Streuung im Neigungswinkel berechnet. Die Ergebnisse bestätigen, dass die schnellen Moden die CR-Ausbreitung dominieren, während Alfv´en langsame Moden bei ähnlichen Neigungswinkelstreuungsraten viel weniger effizient sind. Der Querfeldtransport auf großen und kleinen Skalen wird als nächstes untersucht. Auf großen/globalen Skalen wird normale Diffusion beobachtet und der Diffusionskoeffizient wird durch MζA im Vergleich zu den parallelen Diffusionskoeffizienten unterdrückt, wobei ζin Alfv´en-Moden näher bei 4 liegt als in der Gesamtturbulenz, wie theoretisch erwartet. Für den CR-Transport auf Skalen, die kleiner sind als die Turbulenzinjektionsskala L, werden sowohl der lokale als auch der globale magnetische Bezugsrahmen verwendet. Auf solch kleinen Skalen wird in allen Fällen Superdiffusion beobachtet. Insbesondere der CRTransport in Alfv’en-Moden zeigt eine deutliche Richardson-Diffusion im lokalen Bezugssystem. Die Diffusion geht fließend von der Richardson-Diffusion mit dem Index 1,5 zur normalen Diffusion über, wenn die mittlere freie Weglänge der Teilchen, λ∥, von λ∥ ≫ L auf λ∥ ≪ L abnimmt. Diese Ergebnisse haben eine breite Anwendung auf CRs in verschiedenen astrophysikalischen Umgebungen.
KW  - isotropic fast mode turbulence
KW  - cascade rate
KW  - Alfv´en mode MHD turbulence
KW  - cosmic ray diffusion
KW  - efficient scattering
KW  - mean free path
KW  - Richardson Superdiffusion
KW  - Alfv´en-Modus MHD-Turbulenz
KW  - Richardson-Superdiffusion
KW  - Kaskadenrate
KW  - Diffusion kosmischer Strahlung
KW  - effiziente Streuung
KW  - Isotroper schneller Modus Turbulenzen
KW  - bedeuten freie Bahn
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-589030
ER  - 
TY  - JOUR
A1  - Adolfs, Marjolijn
A1  - Hoque, Mohammed Mainul
A1  - Shprits, Yuri
T1  - Storm-time relative total electron content modelling using machine learning techniques
JF  - Remote sensing
N2  - Accurately predicting total electron content (TEC) during geomagnetic storms is still a challenging task for ionospheric models. In this work, a neural-network (NN)-based model is proposed which predicts relative TEC with respect to the preceding 27-day median TEC, during storm time for the European region (with longitudes 30 degrees W-50 degrees E and latitudes 32.5 degrees N-70 degrees N). The 27-day median TEC (referred to as median TEC), latitude, longitude, universal time, storm time, solar radio flux index F10.7, global storm index SYM-H and geomagnetic activity index Hp30 are used as inputs and the output of the network is the relative TEC. The relative TEC can be converted to the actual TEC knowing the median TEC. The median TEC is calculated at each grid point over the European region considering data from the last 27 days before the storm using global ionosphere maps (GIMs) from international GNSS service (IGS) sources. A storm event is defined when the storm time disturbance index Dst drops below 50 nanotesla. The model was trained with storm-time relative TEC data from the time period of 1998 until 2019 (2015 is excluded) and contains 365 storms. Unseen storm data from 33 storm events during 2015 and 2020 were used to test the model. The UQRG GIMs were used because of their high temporal resolution (15 min) compared to other products from different analysis centers. The NN-based model predictions show the seasonal behavior of the storms including positive and negative storm phases during winter and summer, respectively, and show a mixture of both phases during equinoxes. The model's performance was also compared with the Neustrelitz TEC model (NTCM) and the NN-based quiet-time TEC model, both developed at the German Aerospace Agency (DLR). The storm model has a root mean squared error (RMSE) of 3.38 TEC units (TECU), which is an improvement by 1.87 TECU compared to the NTCM, where an RMSE of 5.25 TECU was found. This improvement corresponds to a performance increase by 35.6%. The storm-time model outperforms the quiet-time model by 1.34 TECU, which corresponds to a performance increase by 28.4% from 4.72 to 3.38 TECU. The quiet-time model was trained with Carrington averaged TEC and, therefore, is ideal to be used as an input instead of the GIM derived 27-day median. We found an improvement by 0.8 TECU which corresponds to a performance increase by 17% from 4.72 to 3.92 TECU for the storm-time model using the quiet-time-model predicted TEC as an input compared to solely using the quiet-time model.
KW  - ionosphere
KW  - relative total electron content
KW  - geomagnetic storms
KW  - neural
KW  - networks
KW  - NTCM
KW  - European storm-time model
Y1  - 2022
U6  - https://doi.org/10.3390/rs14236155
SN  - 2072-4292
VL  - 14
IS  - 23
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Smirnov, Artem
A1  - Berrendorf, Max
A1  - Shprits, Yuri
A1  - Kronberg, Elena A.
A1  - Allison, Hayley J.
A1  - Aseev, Nikita
A1  - Zhelavskaya, Irina
A1  - Morley, Steven K.
A1  - Reeves, Geoffrey D.
A1  - Carver, Matthew R.
A1  - Effenberger, Frederic
T1  - Medium energy electron flux in earth's outer radiation
belt (MERLIN)
BT  - a Machine learning model
JF  - Space weather : the international journal of research and applications
N2  - The radiation belts of the Earth, filled with energetic electrons, comprise complex and dynamic systems that pose a significant threat to satellite operation. While various models of electron flux both for low and relativistic energies have been developed, the behavior of medium energy (120-600 keV) electrons, especially in the MEO region, remains poorly quantified. At these energies, electrons are driven by both convective and diffusive transport, and their prediction usually requires sophisticated 4D modeling codes. In this paper, we present an alternative approach using the Light Gradient Boosting (LightGBM) machine learning algorithm. The Medium Energy electRon fLux In Earth's outer radiatioN belt (MERLIN) model takes as input the satellite position, a combination of geomagnetic indices and solar wind parameters including the time history of velocity, and does not use persistence. MERLIN is trained on >15 years of the GPS electron flux data and tested on more than 1.5 years of measurements. Tenfold cross validation yields that the model predicts the MEO radiation environment well, both in terms of dynamics and amplitudes o f flux. Evaluation on the test set shows high correlation between the predicted and observed electron flux (0.8) and low values of absolute error. The MERLIN model can have wide space weather applications, providing information for the scientific community in the form of radiation belts reconstructions, as well as industry for satellite mission design, nowcast of the MEO environment, and surface charging analysis.
KW  - machine learning
KW  - radiation belts
KW  - electron flux
KW  - empirical modeling
KW  - magnetosphere
KW  - electrons
Y1  - 2020
U6  - https://doi.org/10.1029/2020SW002532
SN  - 1542-7390
VL  - 18
IS  - 11
PB  - American geophysical union, AGU
CY  - Washington
ER  - 
TY  - JOUR
A1  - Cervantes Villa, Juan Sebastian
A1  - Shprits, Yuri
A1  - Aseev, Nikita
A1  - Drozdov, Alexander
A1  - Castillo Tibocha, Angelica Maria
A1  - Stolle, Claudia
T1  - Identifying radiation belt electron source and loss processes by assimilating spacecraft data in a three-dimensional diffusion model
JF  - Journal of geophysical research : Space physics
N2  - Data assimilation aims to blend incomplete and inaccurate data with physics-based dynamical models. In the Earth's radiation belts, it is used to reconstruct electron phase space density, and it has become an increasingly important tool in validating our current understanding of radiation belt dynamics, identifying new physical processes, and predicting the near-Earth hazardous radiation environment. In this study, we perform reanalysis of the sparse measurements from four spacecraft using the three-dimensional Versatile Electron Radiation Belt diffusion model and a split-operator Kalman filter over a 6-month period from 1 October 2012 to 1 April 2013. In comparison to previous works, our 3-D model accounts for more physical processes, namely, mixed pitch angle-energy diffusion, scattering by Electromagnetic Ion Cyclotron waves, and magnetopause shadowing. We describe how data assimilation, by means of the innovation vector, can be used to account for missing physics in the model. We use this method to identify the radial distances from the Earth and the geomagnetic conditions where our model is inconsistent with the measured phase space density for different values of the invariants mu and K. As a result, the Kalman filter adjusts the predictions in order to match the observations, and we interpret this as evidence of where and when additional source or loss processes are active. The current work demonstrates that 3-D data assimilation provides a comprehensive picture of the radiation belt electrons and is a crucial step toward performing reanalysis using measurements from ongoing and future missions.
KW  - acceleration
KW  - code
KW  - density
KW  - emic waves
KW  - energetic particle
KW  - mechanisms
KW  - reanalysis
KW  - ultrarelativistic electrons
KW  - weather
Y1  - 2019
U6  - https://doi.org/10.1029/2019JA027514
SN  - 2169-9380
SN  - 2169-9402
VL  - 125
IS  - 1
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Smirnov, Artem
A1  - Shprits, Yuri
A1  - Allison, Hayley
A1  - Aseev, Nikita
A1  - Drozdov, Alexander
A1  - Kollmann, Peter
A1  - Wang, Dedong
A1  - Saikin, Anthony
T1  - An empirical model of the equatorial electron pitch angle distributions in earth's outer radiation belt
JF  - Space Weather: the International Journal of Research and Applications
N2  - In this study, we present an empirical model of the equatorial electron pitch angle distributions (PADs) in the outer radiation belt based on the full data set collected by the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes in 2012-2019. The PADs are fitted with a combination of the first, third and fifth sine harmonics. The resulting equation resolves all PAD types found in the outer radiation belt (pancake, flat-top, butterfly and cap PADs) and can be analytically integrated to derive omnidirectional flux. We introduce a two-step modeling procedure that for the first time ensures a continuous dependence on L, magnetic local time and activity, parametrized by the solar wind dynamic pressure. We propose two methods to reconstruct equatorial electron flux using the model. The first approach requires two uni-directional flux observations and is applicable to low-PA data. The second method can be used to reconstruct the full equatorial PADs from a single uni- or omnidirectional measurement at off-equatorial latitudes. The model can be used for converting the long-term data sets of electron fluxes to phase space density in terms of adiabatic invariants, for physics-based modeling in the form of boundary conditions, and for data assimilation purposes.
KW  - pitch angle
KW  - radiation belt
KW  - model
KW  - magnetosphere
KW  - van allen probes;
KW  - electrons
Y1  - 2022
U6  - https://doi.org/10.1029/2022SW003053
SN  - 1542-7390
VL  - 20
IS  - 9
PB  - American Geophysical Union
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Walker, Simon N.
A1  - Boynton, Richard J.
A1  - Shprits, Yuri
A1  - Balikhin, Michael A.
A1  - Drozdov, Alexander
T1  - Forecast of the energetic electron environment of the radiation belts
JF  - Space Weather: The International Journal of Research and Applications
N2  - Different modeling methodologies possess different strengths and weakness. For instance, data based models may provide superior accuracy but have a limited spatial coverage while physics based models may provide lower accuracy but provide greater spatial coverage. This study investigates the coupling of a data based model of the electron fluxes at geostationary orbit (GEO) with a numerical model of the radiation belt region to improve the resulting forecasts/pastcasts of electron fluxes over the whole radiation belt region. In particular, two coupling methods are investigated. The first assumes an average value for L* for GEO, namely LGEO* L-GEO* = 6.2. The second uses a value of L* that varies with geomagnetic activity, quantified using the Kp index. As the terrestrial magnetic field responds to variations in geomagnetic activity, the value of L* will vary for a specific location. In this coupling method, the value of L* is calculated using the Kp driven Tsyganenko 89c magnetic field model for field line tracing. It is shown that this addition can result in changes in the initialization of the parameters at the Versatile Electron Radiation Belt model outer boundary. Model outputs are compared to Van Allen Probes MagEIS measurements of the electron fluxes in the inner magnetosphere for the March 2015 geomagnetic storm. It is found that the fixed LGEO* L-GEO* coupling method produces a more realistic forecast.
KW  - radiation belt forecasts
KW  - data based NARMAX modeling
KW  - verb simulations;
KW  - geostationary orbit
KW  - electron flux forecasts
Y1  - 2022
U6  - https://doi.org/10.1029/2022SW003124
SN  - 1542-7390
VL  - 20
IS  - 12
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Prol, Fabricio S.
A1  - Smirnov, Artem G.
A1  - Hoque, M. Mainul
A1  - Shprits, Yuri
T1  - Combined model of topside ionosphere and plasmasphere derived from radio-occultation and Van Allen Probes data
JF  - Scientific reports
N2  - In the last years, electron density profile functions characterized by a linear dependence on the scale height showed good results when approximating the topside ionosphere. The performance above 800 km, however, is not yet well investigated. 
This study investigates the capability of the semi-Epstein functions to represent electron density profiles from the peak height up to 20,000 km. Electron density observations recorded by the Van Allen Probes were used to resolve the scale height dependence in the plasmasphere. 
It was found that the linear dependence of the scale height in the topside ionosphere cannot be directly used to extrapolate profiles above 800 km. 
We find that the dependence of scale heights on altitude is quadratic in the plasmasphere. A statistical model of the scale heights is therefore proposed. After combining the topside ionosphere and plasmasphere by a unified model, we have obtained good estimations not only in the profile shapes, but also in the Total Electron Content magnitude and distributions when compared to actual measurements from 2013, 2014, 2016 and 2017. 
Our investigation shows that Van Allen Probes can be merged to radio-occultation data to properly represent the upper ionosphere and plasmasphere by means of a semi-Epstein function.
Y1  - 2022
U6  - https://doi.org/10.1038/s41598-022-13302-1
SN  - 2045-2322
VL  - 12
IS  - 1
PB  - Macmillan Publishers Limited, part of Springer Nature
CY  - London
ER  - 
TY  - JOUR
A1  - Haas, Bernhard
A1  - Shprits, Yuri
A1  - Allison, Hayley
A1  - Wutzig, Michael
A1  - Wang, Dedong
T1  - Which parameter controls ring current electron dynamics
JF  - Frontiers in astronomy and space sciences
N2  - Predicting the electron population of Earth's ring current during geomagnetic storms still remains a challenging task. 
In this work, we investigate the sensitivity of 10 keV ring current electrons to different driving processes, parameterised by the Kp index, during several moderate and intense storms. 
Results are validated against measurements from the Van Allen Probes satellites. Perturbing the Kp index allows us to identify the most dominant processes for moderate and intense storms respectively. 
We find that during moderate storms (Kp < 6) the drift velocities mostly control the behaviour of low energy electrons, while loss from wave-particle interactions is the most critical parameter for quantifying the evolution of intense storms (Kp > 6). Perturbations of the Kp index used to drive the boundary conditions at GEO and set the plasmapause location only show a minimal effect on simulation results over a limited L range. 
It is further shown that the flux at L & SIM; 3 is more sensitive to changes in the Kp index compared to higher L shells, making it a good proxy for validating the source-loss balance of a ring current model.
KW  - ring current
KW  - magnetosphere
KW  - electron lifetimes
KW  - electrons
KW  - van allen probes (RBSP)
KW  - ring current model
KW  - verb
Y1  - 2022
U6  - https://doi.org/10.3389/fspas.2022.911002
SN  - 2296-987X
VL  - 9
PB  - Frontiers Media
CY  - Lausanne
ER  - 
TY  - THES
A1  - Aseev, Nikita
T1  - Modeling and understanding dynamics of charged particles in the Earth's inner magnetosphere
T1  - Modellierung und Untersuchung der Dynamik geladener Teilchen in der inneren Magnetosphäre der Erde
N2  - The Earth's inner magnetosphere is a very dynamic system, mostly driven by the external solar wind forcing exerted upon the magnetic field of our planet. Disturbances in the solar wind, such as coronal mass ejections and co-rotating interaction regions, cause geomagnetic storms, which lead to prominent changes in charged particle populations of the inner magnetosphere - the plasmasphere, ring current, and radiation belts. Satellites operating in the regions of elevated energetic and relativistic electron fluxes can be damaged by deep dielectric or surface charging during severe space weather events. Predicting the dynamics of the charged particles and mitigating their effects on the infrastructure is of particular importance, due to our increasing reliance on space technologies.

The dynamics of particles in the plasmasphere, ring current, and radiation belts are strongly coupled by means of collisions and collisionless interactions with electromagnetic fields induced by the motion of charged particles. Multidimensional numerical models simplify the treatment of transport, acceleration, and loss processes of these particles, and allow us to predict how the near-Earth space environment responds to solar storms. The models inevitably rely on a number of simplifications and assumptions that affect model accuracy and complicate the interpretation of the results. In this dissertation, we quantify the processes that control electron dynamics in the inner magnetosphere, paying particular attention to the uncertainties of the employed numerical codes and tools.

We use a set of convenient analytical solutions for advection and diffusion equations to test the accuracy and stability of the four-dimensional Versatile Electron Radiation Belt (VERB-4D) code. We show that numerical schemes implemented in the code converge to the analytical solutions and that the VERB-4D code demonstrates stable behavior independent of the assumed time step. The order of the numerical scheme for the convection equation is demonstrated to affect results of ring current and radiation belt simulations, and it is crucially important to use high-order numerical schemes to decrease numerical errors in the model.

Using the thoroughly tested VERB-4D code, we model the dynamics of the ring current electrons during the 17 March 2013 storm. The discrepancies between the model and observations above 4.5 Earth's radii can be explained by uncertainties in the outer boundary conditions. Simulation results indicate that the electrons were transported from the geostationary orbit towards the Earth by the global-scale electric and magnetic fields.
We investigate how simulation results depend on the input models and parameters. The model is shown to be particularly sensitive to the global electric field and electron lifetimes below 4.5 Earth's radii. The effects of radial diffusion and subauroral polarization streams are also quantified.

We developed a data-assimilative code that blends together a convection model of energetic electron transport and loss and Van Allen Probes satellite data by means of the Kalman filter. We show that the Kalman filter can correct model uncertainties in the convection electric field, electron lifetimes, and boundary conditions. It is also demonstrated how the innovation vector - the difference between observations and model prediction - can be used to identify physical processes missing in the model of energetic electron dynamics.

We computed radial profiles of phase space density of ultrarelativistic electrons, using Van Allen Probes measurements. We analyze the shape of the profiles during geomagnetically quiet and disturbed times and show that the formation of new local minimums in the radial profiles coincides with the ground observations of electromagnetic ion-cyclotron (EMIC) waves. This correlation indicates that EMIC waves are responsible for the loss of ultrarelativistic electrons from the heart of the outer radiation belt into the Earth's atmosphere.
N2  - Die innere Magnetosphäre der Erde ist ein sehr dynamisches System, das hauptsächlich vom äußeren Sonnenwind beeinflusst wird, der auf das Magnetfeld unseres Planeten einwirkt. Störungen im Sonnenwind, wie z.B. koronale Massenauswürfe und sogenannte Korotierende Wechselwirkungsbereiche, verursachen geomagnetische Stürme, die zu deutlichen Veränderungen der Populationen geladener Teilchen in der inneren Magnetosphäre führen - Plasmasphäre, Ringstrom und Strahlungsgürtel. Satelliten, die in Regionen mit erhöhten energetischen und relativistischen Elektronenflüssen betrieben werden, können durch tiefe dielektrische Ladung oder Oberflächenladungen bei schweren Weltraumwetterereignissen beschädigt werden. Die Vorhersage der Dynamik der geladenen Teilchen und die Abschwächung ihrer Auswirkungen auf die Infrastruktur sind heutzutage von besonderer Bedeutung, insbesondere aufgrund unserer zunehmenden Abhängigkeit von Weltraumtechnologien.

Die Dynamik von Teilchen in der Plasmasphäre, des Ringstrom und in den Strahlungsgürteln sind durch Kollisionen und kollisionsfreie Wechselwirkungen mit elektromagnetischen Feldern, die durch die Bewegung geladener Teilchen induziert werden, stark gekoppelt. Mehrdimensionale numerische Modelle vereinfachen die Betrachtung von Transport-, Beschleunigungs- und Verlustprozessen dieser Partikel und ermöglichen es uns, vorherzusagen, wie die erdnahe Weltraumumgebung auf Sonnenstürme reagiert. Die Modelle beruhen zwangsläufig auf einer Reihe von Vereinfachungen und Voraussetzungen, die sich auf die Modellgenauigkeit auswirken und die Interpretation der Ergebnisse erschweren. In dieser Dissertation quantifizieren wir die Prozesse, die die Dynamik der Elektronen in der inneren Magnetosphäre steuern. Dabei richten wir den Fokus insbesondere auch auf die Unsicherheiten der verwendeten numerischen Codes.

Wir verwenden eine Reihe praktischer analytischer Lösungen für Advektions- und Diffusionsgleichungen, um die Genauigkeit und Stabilität des 4-dimensionalen ''Versatile Electron Radiation Belt'' Codes (VERB-4D Code) zu testen. Wir zeigen, dass die im Code implementierten numerischen Schemata zu den analytischen Lösungen konvergieren und der Code sich unabhängig vom angenommenen Zeitschritt stabil verhält. Wir demonstrieren, wie die Genauigkeit des numerischen Schemas für die Konvektionsgleichung die Ergebnisse von Ringstrom- und Strahlungsgürtelsimulationen beeinflussen kann, und dass es von entscheidender Beteutung ist, numerische Schemata höherer Ordnung zu verwenden, um numerische Fehler im Modell zu reduzieren.

Mit dem ausführlich getesteten VERB-4D Code modellieren wir die Dynamik der Ringstromelektronen während des Sturms vom 17. März 2013. Wir zeigen, dass die Diskrepanzen zwischen dem Modell und Beobachtungen oberhalb von 4.5 Erdradien durch Unsicherheiten in den äußeren Randbedingungen erklärt werden können und dass die Elektronen durch die globalen elektrischen und magnetischen Felder von der geostationäre Umlaufbahn zur Erde transportiert wurden.
Wir untersuchen weiterhin, wie die Simulationsergebnisse von den Eingabemodellen und Parametern abhängen. Wir zeigen, dass das Modell besonders empfindlich für das globale elektrische Feld und die Lebensdauer der Elektronen unterhalb von 4.5 Erdradien ist. Außerdem quantifizieren wir auch die Auswirkungen von radialer Diffusion und subauroralen Polarisationsströmen.

Wir haben einen datenassimilativen Code entwickelt, der mithilfe des Kalman-Filters ein Konvektionsmodell für den Transport und den Verlust energetischer Elektronen mit den Satellitendaten der Van Allen Probes kombiniert. Wir zeigen, dass die Verwendung eines Kalman-Filters Modellunsicherheiten im elektrischen Konvektionsfeld, in der Lebensdauer der Elektronen und in den Randbedingungen korrigieren kann. Weiterhin zeigen wir, wie der Innovationsvektor - die Differenz zwischen Beobachtungen und Modellvorhersagen - verwendet werden kann, um physikalische Prozesse zu identifizieren, die im Modell der Dynamik der energetischen Elektronen fehlen.

Außerdem berechnen wir radiale Profile der Phasenraumdichte ultrarelativistischer Elektronen mithilfe von Van Allen Probes-Messungen. Wir analysieren die Form der Profile und zeigen, dass die Entstehung neuer lokaler Minima in den radialen Profilen mit den Bodenbeobachtungen von EMIC-Wellen übereinstimmt. Diese Korrelation legt nahe, dass EMIC-Wellen für den Verlust ultrarelativistischer Elektronen vom Herzen des äußeren Strahlungsgürtels in die Erdatmosphäre verantwortlich sind.
KW  - ring current electrons
KW  - radiation belts
KW  - mathematical modeling
KW  - wave-particle interactions
KW  - data assimilation
KW  - Ringstromelektronen
KW  - Strahlungsgürtel
KW  - mathematische Modellierung
KW  - Wellen-Teilchen Wechselwirkungen
KW  - Datenassimilation
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-479211
ER  - 
TY  - JOUR
A1  - Kim, Kyung-Chan
A1  - Shprits, Yuri
T1  - Statistical Analysis of Hiss Waves in Plasmaspheric Plumes Using Van Allen Probe Observations
JF  - Journal of geophysical research : Space physics
N2  - Plasmaspheric hiss waves commonly observed in high‐density regions in the Earth's magnetosphere are known to be one of the main contributors to the loss of radiation belt electrons. There has been a lot of effort to investigate the distributions of hiss waves in the plasmasphere, while relatively little attention has been given to those in the plasmaspheric plume. In this study, we present for the first time a statistical analysis of the occurrence and the spatial distribution of wave amplitudes and wave normal angles for hiss waves in plumes using Van Allen Probes observations during the period of October 2012 to December 2016. Statistical results show that a wide range of hiss wave amplitudes in plumes from a few picotesla to >100 pT is observed, but a modest (<20 pT) wave amplitude is more commonly observed regardless of geomagnetic activity in both the midnight‐to‐dawn and dusk sector. By contrast, stronger amplitude hiss occurs preferentially during geomagnetically active times in the dusk sector. The wave normal angles are distributed over a broad range from 0° to 90° with a bimodal distribution: a quasi‐field‐aligned population (<20°) with an occurrence rate of <60% and an oblique one (>50°) with a relative low occurrence rate of ≲20%. Therefore, from a statistical point of view, we confirm that the hiss intensity (a few tens of picotesla) and field‐aligned hiss wave adopted in previous simulation studies are a reasonable assumption but stress that the activity dependence of the wave amplitude should be considered.
KW  - plasmaspheric hiss
KW  - plasmaspheric plume
KW  - Van Allen Probes
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026458
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 3
SP  - 1904
EP  - 1915
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Zhu, Hui
A1  - Shprits, Yuri
A1  - Spasojevic, M.
A1  - Drozdov, Alexander
T1  - New hiss and chorus waves diffusion coefficient parameterizations from the Van Allen Probes and their effect on long-term relativistic electron radiation-belt VERB simulations
JF  - Journal of Atmospheric and Solar-Terrestrial Physics
N2  - New wave frequency and amplitude models for the nightside and dayside chorus waves are built based on measurements from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument onboard the Van Allen Probes. The corresponding 3D diffusion coefficients are systematically obtained. Compared with previous commonly-used (typical) parameterizations, the new parameterizations result in differences in diffusion rates that depend on the energy and pitch angle. Furthermore, one-year 3D diffusive simulations are performed using the Versatile Electron Radiation Belt (VERB) code. Both typical and new wave parameterizations simulation results are in a good agreement with observations at 0.9 MeV. However, the new parameterizations for nightside chorus better reproduce the observed electron fluxes. These parameterizations will be incorporated into future modeling efforts.
KW  - Inner magnetosphere
KW  - Radiation belts
KW  - Chorus waves
KW  - Diffusion coefficients
KW  - VERB code
Y1  - 2019
U6  - https://doi.org/10.1016/j.jastp.2019.105090
SN  - 1364-6826
SN  - 1879-1824
VL  - 193
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Drozdov, Alexander
A1  - Aseev, Nikita
A1  - Effenberger, Frederic
A1  - Turner, Drew L.
A1  - Saikin, Anthony
A1  - Shprits, Yuri
T1  - Storm Time Depletions of Multi-MeV Radiation Belt Electrons Observed at Different Pitch Angles
JF  - Journal of geophysical research : Space physics
N2  - During geomagnetic storms, the rapid depletion of the high-energy (several MeV) outer radiation belt electrons is the result of loss to the interplanetary medium through the magnetopause, outward radial diffusion, and loss to the atmosphere due to wave-particle interactions. We have performed a statistical study of 110 storms using pitch angle resolved electron flux measurements from the Van Allen Probes mission and found that inside of the radiation belt (L* = 3 - 5) the number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 30 degrees is higher than number of storms that result in depletion of electrons with equatorial pitch angle alpha(eq) = 75 degrees. We conclude that this result is consistent with electron scattering by whistler and electromagnetic ion cyclotron waves. At the outer edge of the radiation belt (L* >= 5.2) the number of storms that result in depletion is also large (similar to 40-50%), emphasizing the significance of the magnetopause shadowing effect and outward radial transport.
Y1  - 2019
U6  - https://doi.org/10.1029/2019JA027332
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 11
SP  - 8943
EP  - 8953
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Castillo, Angelica M.
A1  - Shprits, Yuri
A1  - Ganushkina, Natalia
A1  - Drozdov, Alexander
A1  - Aseev, Nikita
A1  - Wang, Dedong
A1  - Dubyagin, Stepan
T1  - Simulations of the inner magnetospheric energetic electrons using the IMPTAM-VERB coupled model
JF  - Journal of Atmospheric and Solar-Terrestrial Physics
N2  - In this study, we present initial results of the coupling between the Inner Magnetospheric Particle Transport and Acceleration Model (IMPTAM) and the Versatile Electron Radiation Belt (VERB-3D) code. IMPTAM traces electrons of 10-100 keV energies from the plasma sheet (L = 9 Re) to inner L-shell regions. The flux evolution modeled by IMPTAM is used at the low energy and outer L* computational boundaries of the VERB code (assuming a dipole approximation) to perform radiation belt simulations of energetic electrons. The model was tested on the March 17th, 2013 storm, for a six-day period. Four different simulations were performed and their results compared to satellites observations from Van Allen probes and GOES. The coupled IMPTAM-VERB model reproduces evolution and storm-time features of electron fluxes throughout the studied storm in agreement with the satellite data (within similar to 0.5 orders of magnitude). Including dynamics of the low energy population at L* = 6.6 increases fluxes closer to the heart of the belt and has a strong impact in the VERB simulations at all energies. However, inclusion of magnetopause losses leads to drastic flux decreases even below L* = 3. The dynamics of low energy electrons (max. 10s of keV) do not affect electron fluxes at energies >= 900 keV. Since the IMPTAM-VERB coupled model is only driven by solar wind parameters and the Dst and Kp indexes, it is suitable as a forecasting tool. In this study, we demonstrate that the estimation of electron dynamics with satellite-data-independent models is possible and very accurate.
KW  - Electron populations
KW  - Radiation belts
KW  - IMPTAM
KW  - VERB
Y1  - 2019
U6  - https://doi.org/10.1016/j.jastp.2019.05.014
SN  - 1364-6826
SN  - 1879-1824
VL  - 191
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Woodfield, Emma E.
A1  - Glauert, Saraha A.
A1  - Menietti, J. Douglas
A1  - Averkamp, Terrance F.
A1  - Horne, Richard B.
A1  - Shprits, Yuri
T1  - Rapid Electron Acceleration in Low‐Density Regions of Saturn's Radiation Belt by Whistler Mode Chorus Waves
JF  - Geophysical research letters
N2  - Electron acceleration at Saturn due to whistler mode chorus waves has previously been assumed to be ineffective; new data closer to the planet show it can be very rapid (factor of 104 flux increase at 1 MeV in 10 days compared to factor of 2). A full survey of chorus waves at Saturn is combined with an improved plasma density model to show that where the plasma frequency falls below the gyrofrequency additional strong resonances are observed favoring electron acceleration. This results in strong chorus acceleration between approximately 2.5 R-S and 5.5 R-S outside which adiabatic transport may dominate. Strong pitch angle dependence results in butterfly pitch angle distributions that flatten over a few days at 100s keV, tens of days at MeV energies which may explain observations of butterfly distributions of MeV electrons near L = 3. Including cross terms in the simulations increases the tendency toward butterfly distributions. Plain Language Summary Radiation belts are hazardous regions found around several of the planets in our Solar System. They consist of very hot, electrically charged particles trapped in the magnetic field of the planet. At Saturn the most important way to heat these particles has for many years been thought to involve the particles drifting closer toward the planet. This paper adds to the emerging idea at Saturn that a different way to heat the particles is also possible where the heating is done by waves, in a similar way to what we find at the Earth. We use recent information from the Cassini spacecraft on the number and location of particles and also of the waves strength and location combined with computer simulations to show that a particular wave called chorus is excellent at heating the particles where the surrounding number of cold particles is low.
Y1  - 2019
U6  - https://doi.org/10.1029/2019GL083071
SN  - 0094-8276
SN  - 1944-8007
VL  - 46
IS  - 13
SP  - 7191
EP  - 7198
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Denton, Richard E.
A1  - Ofman, L.
A1  - Shprits, Yuri
A1  - Bortnik, J.
A1  - Millan, R. M.
A1  - Rodger, C. J.
A1  - da Silva, C. L.
A1  - Rogers, B. N.
A1  - Hudson, M. K.
A1  - Liu, K.
A1  - Min, K.
A1  - Glocer, A.
A1  - Komar, C.
T1  - Pitch Angle Scattering of Sub-MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves
JF  - Journal of geophysical research : Space physics
N2  - Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low-amplitude short-wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub-megaelectron volts (sub-MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves.
KW  - electromagnetic ion cyclotron waves
KW  - EMIC
KW  - relativistic electron precipitation
KW  - pitch angle scattering
KW  - wave particle interaction
KW  - radiation belts
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026384
SN  - 2169-9402
VL  - 124
IS  - 7
SP  - 5610
EP  - 5626
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Qin, Murong
A1  - Hudson, Mary
A1  - Li, Zhao
A1  - Millan, Robyn
A1  - Shen, Xiaochen
A1  - Shprits, Yuri
A1  - Woodger, Leslie
A1  - Jaynes, Allison
A1  - Kletzing, Craig
T1  - Investigating loss of relativistic electrons associated with EMIC Waves at low L values on 22 June 2015
JF  - Journal of geophysical research : Space physics
N2  - In this study, rapid loss of relativistic radiation belt electrons at low L* values (2.4-3.2) during a strong geomagnetic storm on 22 June 2015 is investigated along with five possible loss mechanisms. Both the particle and wave data are obtained from the Van Allen Probes. Duskside H+ band electromagnetic ion cyclotron (EMIC) waves were observed during a rapid decrease of relativistic electrons with energy above 5.2 MeV occurring outside the plasma sphere during extreme magnetopause compression. Lower He+ composition and enriched O+ composition are found compared to typical values assumed in other studies of cyclotron resonant scattering of relativistic electrons by EMIC waves. Quantitative analysis demonstrates that even with the existence of He+ band EMIC waves, it is the H+ band EMIC waves that are likely to cause the depletion at small pitch angles and strong gradients in pitch angle distributions of relativistic electrons with energy above 5.2 MeV at low L values for this event. Very low frequency wave activity at other magnetic local time can be favorable for the loss of relativistic electrons at higher pitch angles. An illustrative calculation that combines the nominal pitch angle scattering rate due to whistler mode chorus at high pitch angles with the H+ band EMIC wave loss rate at low pitch angles produces loss on time scale observed at L = 2.4-3.2. At high L values and lower energies, radial loss to the magnetopause is a viable explanation.
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA025726
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 6
SP  - 4022
EP  - 4036
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Shprits, Yuri
A1  - Vasile, Ruggero
A1  - Zhelayskaya, Irina S.
T1  - Nowcasting and Predicting the Kp Index Using Historical Values and Real-Time Observations
JF  - Space Weather: The International Journal of Research and Applications
N2  - Current algorithms for the real-time prediction of the Kp index use a combination of models empirically driven by solar wind measurements at the L1 Lagrange point and historical values of the index. In this study, we explore the limitations of this approach, examining the forecast for short and long lead times using measurements at L1 and Kp time series as input to artificial neural networks. We explore the relative efficiency of the solar wind-based predictions, predictions based on recurrence, and predictions based on persistence. Our modeling results show that for short-term forecasts of approximately half a day, the addition of the historical values of Kp to the measured solar wind values provides a barely noticeable improvement. For a longer-term forecast of more than 2 days, predictions can be made using recurrence only, while solar wind measurements provide very little improvement for a forecast with long horizon times. We also examine predictions for disturbed and quiet geomagnetic activity conditions. Our results show that the paucity of historical measurements of the solar wind for high Kp results in a lower accuracy of predictions during disturbed conditions. Rebalancing of input data can help tailor the predictions for more disturbed conditions.
KW  - Kp index
KW  - geomagnetic activity
KW  - empirical prediction
KW  - solar wind
KW  - forecast
KW  - AI
Y1  - 2019
U6  - https://doi.org/10.1029/2018SW002141
SN  - 1542-7390
VL  - 17
IS  - 8
SP  - 1219
EP  - 1229
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Postnov, K.
A1  - Oskinova, Lidia M.
A1  - Torrejon, J. M.
T1  - A propelling neutron star in the enigmatic Be-star gamma Cassiopeia
JF  - Monthly notices of the Royal Astronomical Society
N2  - gamma Cassiopeia (gamma Cas), is known to be a binary system consisting of a Be-type star and a low-mass (M similar to 1M(circle dot)) companion of unknown nature orbiting in the Be-disc plane. Here, we apply the quasi-spherical accretion theory on to a compact magnetized star and show that if the low-mass companion of gamma Cas is a fast spinning neutron star, the key observational signatures of. Cas are remarkably well reproduced. Direct accretion on to this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed which emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from gamma Cas. We suggest that gamma Cas and its analogues constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of gamma Cas and its analogues should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from gamma Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.
KW  - stars: emission-line, Be
KW  - stars: neutron
Y1  - 2017
U6  - https://doi.org/10.1093/mnrasl/slw223
SN  - 0035-8711
SN  - 1365-2966
VL  - 465
IS  - 1
SP  - L119
EP  - L123
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Markötter, Henning
A1  - Sintschuk, Michael
A1  - Britzke, Ricardo
A1  - Dayani, Shahabeddin
A1  - Bruno, Giovanni
T1  - Upgraded imaging capabilities at the BAMline (BESSY II)
JF  - Journal of synchrotron radiation
N2  - The BAMline at the BESSY II synchrotron X-ray source has enabled research for more than 20 years in widely spread research fields such as materials science, biology, cultural heritage and medicine. 
As a nondestructive characterization method, synchrotron X-ray imaging, especially tomography, plays a particularly important role in structural characterization. 
A recent upgrade of key equipment of the BAMline widens its imaging capabilities: shorter scan acquisition times are now possible, in situ and op erando studies can now be routinely performed, and different energy spectra can easily be set up. 
In fact, the upgraded double-multilayer monochromator brings full flexibility by yielding different energy spectra to optimize flux and energy resolution as desired. 
The upgraded detector (based on an sCMOS camera) also allows exploiting the higher flux with reduced readout times. 
Furthermore, an installed slip ring allows the sample stage to continuously rotate. 
The latter feature enables tomographic observation of processes occurring in the time scale of a few seconds.
KW  - synchrotron radiation
KW  - computed tomography
KW  - double-multilayer  monochromators
KW  - pink beams
KW  - X-ray optics
Y1  - 2022
U6  - https://doi.org/10.1107/S1600577522007342
SN  - 1600-5775
VL  - 29
IS  - 5
SP  - 1292
EP  - 1298
PB  - International Union of Crystallography
CY  - Chester
ER  - 
TY  - JOUR
A1  - Prüfert, Christian
A1  - Beitz, Toralf
A1  - Reich, Olaf
A1  - Löhmannsröben, Hans-Gerd
T1  - Inline process analysis of copper-bearing aerosols using laser-induced breakdown spectroscopy, laser-induced incandescence and optical imaging
JF  - Spectrochimica acta, Part B, Atomic spectroscopy
N2  - The quantification and identification of aerosols in industry plays a key role in process monitoring and control and lays the foundation for process automation aspired by the industry 4.0 initiative. 
However, measuring particulate matter's mass and number concentrations in harsh environments poses great analytical constraints. 
The presented approach comprises a comprehensive set of light-and imaging-based techniques, all contactless, in-line, and real-time. It includes, but is not limited to, stroboscopic imaging, laser-induced breakdown spectroscopy (LIBS) and laser-induced incandescence (LII). Stroboscopic imaging confirmed the particles sphericity and was used to measure the particle number density. A phase-selective LIBS setup with low fluence and 500 Hz repetition rate was used to classify each particle with a single-pulse and in real time. Simultaneously, the created plasma was captured by CCD imaging to determine the detection volume and hit rate of the LIBS setup. 
Both data sets combined were converted to a particle number density, which was consistent with the particle number density of the stroboscopic measurements. Furthermore, using a photodiode and microphone in parallel to the LIBS setup allowed for the photoacoustic normalization of the spectral line intensity at the laser repetition rate of 500 Hz. 
This was done as a partial photoacoustic normalization method with the cut-off based on the coefficient of variation (CV), reducing it by 25%. Aside from that photodiode and microphone were proven to be valuable event counting with the advantage of the less spatially constricted. A second laser setup was used for laser -induced incandescence (LII) making it possible to classify the particles based on their incandescence tendency. Given its larger probing volume, LII could be employed at very low particle number densities. 
With respect to the current literature, this is the first approach of using LII as an in-line, real-time analytical technique for the compositional classification of metal-bearing aerosols.
KW  - LIBS
KW  - LII
KW  - aerosol. photoacoustics
KW  - stroboscopic imaging
KW  - minerals
Y1  - 2022
U6  - https://doi.org/10.1016/j.sab.2022.106527
SN  - 0584-8547
SN  - 1873-3565
VL  - 197
PB  - Elsevier
CY  - Amsterdam [u.a.]
ER  - 
TY  - JOUR
A1  - Shenar, Tomer
A1  - Sablowski, D. P.
A1  - Hainich, Rainer
A1  - Todt, Helge Tobias
A1  - Moffat, Anthony F. J.
A1  - Oskinova, Lidia M.
A1  - Ramachandran, Varsha
A1  - Sana, Hugues
A1  - Sander, Andreas Alexander Christoph
A1  - Schnurr, O.
A1  - St-Louis, N.
A1  - Vanbeveren, D.
A1  - Gotberg, Y.
A1  - Hamann, Wolf-Rainer
T1  - The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud Spectroscopy, orbital analysis, formation, and evolution
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy budget of galaxies and probe a critical phase in the evolution of massive stars prior to core collapse. It is not known whether core He-burning WR stars (classical WR; cWR) form predominantly through wind stripping (w-WR) or binary stripping (b-WR). Whereas spectroscopy of WR binaries has so-far largely been avoided because of its complexity, our study focuses on the 44 WR binaries and binary candidates of the Large Magellanic Cloud (LMC; metallicity Z approximate to 0.5 Z(circle dot)), which were identified on the basis of radial velocity variations, composite spectra, or high X-ray luminosities. Aims. Relying on a diverse spectroscopic database, we aim to derive the physical and orbital parameters of our targets, confronting evolution models of evolved massive stars at subsolar metallicity and constraining the impact of binary interaction in forming these stars. Methods. Spectroscopy was performed using the Potsdam Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement was performed using the code Spectangular or the shift-and-add algorithm. Evolutionary status was interpreted using the Binary Population and Spectral Synthesis (BPASS) code, exploring binary interaction and chemically homogeneous evolution. Results. Among our sample, 28/44 objects show composite spectra and are analyzed as such. An additional five targets show periodically moving WR primaries but no detected companions (SB1); two (BAT99 99 and 112) are potential WR + compact-object candidates owing to their high X-ray luminosities. We cannot confirm the binary nature of the remaining 11 candidates. About two-thirds of the WN components in binaries are identified as cWR, and one-third as hydrogen-burning WR stars. We establish metallicity-dependent mass-loss recipes, which broadly agree with those recently derived for single WN stars, and in which so-called WN3/O3 stars are clear outliers. We estimate that 45 +/- 30% of the cWR stars in our sample have interacted with a companion via mass transfer. However, only approximate to 12 +/- 7% of the cWR stars in our sample naively appear to have formed purely owing to stripping via a companion (12% b-WR). Assuming that apparently single WR stars truly formed as single stars, this comprises approximate to 4% of the whole LMC WN population, which is about ten times less than expected. No obvious differences in the properties of single and binary WN stars, whose luminosities extend down to log L approximate to 5.2 [L-circle dot], are apparent. With the exception of a few systems (BAT99 19, 49, and 103), the equatorial rotational velocities of the OB-type companions are moderate (v(eq) less than or similar to 250 km s(-1)) and challenge standard formalisms of angular-momentum accretion. For most objects, chemically homogeneous evolution can be rejected for the secondary, but not for the WR progenitor. Conclusions. No obvious dichotomy in the locations of apparently single and binary WN stars on the Hertzsprung-Russell diagram is apparent. According to commonly used stellar evolution models (BPASS, Geneva), most apparently single WN stars could not have formed as single stars, implying that they were stripped by an undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing (e.g., during the red supergiant phase) are strongly underestimated in standard stellar evolution models.
KW  - stars: massive
KW  - stars: Wolf-Rayet
KW  - Magellanic Clouds
KW  - binaries: close
KW  - binaries: spectroscopic
KW  - stars: evolution
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201935684
SN  - 0004-6361
SN  - 1432-0746
VL  - 627
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Shenar, Tomer
A1  - Hainich, Rainer
A1  - Todt, Helge Tobias
A1  - Moffat, Anthony F. J.
A1  - Sander, Andreas Alexander Christoph
A1  - Oskinova, Lidia M.
A1  - Ramachandran, Varsha
A1  - Munoz, M.
A1  - Pablo, H.
A1  - Sana, Hugues
A1  - Hamann, Wolf-Rainer
T1  - The shortest-period Wolf-Rayet binary in the small magellanic cloud
BT  - Part of a high-order multiple system Spectral and orbital analysis of SMC AB 6
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. SMC AB6 is the shortest-period (P = 6.5 d) Wolf-Rayet (WR) binary in the Small Magellanic Cloud. This binary is therefore a key system in the study of binary interaction and formation of WR stars at low metallicity. The WR component in AB6 was previously found to be very luminous (log L = 6.3 [L-circle dot]) compared to its reported orbital mass (approximate to 8 M-circle dot), placing it significantly above the Eddington limit. Aims. Through spectroscopy and orbital analysis of newly acquired optical data taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), we aim to understand the peculiar results reported for this system and explore its evolutionary history. Methods. We measured radial velocities via cross-correlation and performed a spectral analysis using the Potsdam Wolf-Rayet model atmosphere code. The evolution of the system was analyzed using the Binary Population and Spectral Synthesis evolution code. Results. AB6 contains at least four stars. The 6.5 d period WR binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating (v(eq) = 265 km s(-1)) early O-type companion (O5.5 V, star B). Static N III and N IV emission lines and absorption signatures in He lines suggest the presence of an early-type emission line star (O5.5 I(f), star C). Finally, narrow absorption lines portraying a long-term radial velocity variation show the existence of a fourth star (O7.5 V, star D). Star D appears to form a second 140 d period binary together with a fifth stellar member, which is a B-type dwarf or a black hole. It is not clear that these additional components are bound to the WR binary. We derive a mass ratio of M-O/M-WR = 2.2 +/- 0.1. The WR star is found to be less luminous than previously thought (log L = 5.9 [L-circle dot]) and, adopting M-O = 41 M-circle dot for star B, more massive (M-WR = 18 M-circle dot). Correspondingly, the WR star does not exceed the Eddington limit. We derive the initial masses of M-i,M-WR = 60 M-circle dot and M-i,M-O = 40 M-circle dot and an age of 3.9 Myr for the system. The WR binary likely experienced nonconservative mass transfer in the past supported by the relatively rapid rotation of star B. Conclusions. Our study shows that AB6 is a multiple - probably quintuple - system. This finding resolves the previously reported puzzle of the WR primary exceeding the Eddington limit and suggests that the WR star exchanged mass with its companion in the past.
KW  - stars: massive
KW  - binaries: spectroscopic
KW  - stars: Wolf-Rayet
KW  - Magellanic Clouds
KW  - stars: individual: SMC AB 6
KW  - stars: atmospheres
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833006
SN  - 1432-0746
SN  - 0004-6361
VL  - 616
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Shenar, Tomer
A1  - Richardson, N. D.
A1  - Sablowski, Daniel P.
A1  - Hainich, Rainer
A1  - Sana, H.
A1  - Moffat, A. F. J.
A1  - Todt, Helge Tobias
A1  - Hamann, Wolf-Rainer
A1  - Oskinova, Lidia M.
A1  - Sander, Andreas Alexander Christoph
A1  - Tramper, Frank
A1  - Langer, Norbert
A1  - Bonanos, Alceste Z.
A1  - de Mink, Selma E.
A1  - Gräfener, G.
A1  - Crowther, Paul
A1  - Vink, J. S.
A1  - Almeida, Leonardo A.
A1  - de Koter, A.
A1  - Barbá, Rodolfo
A1  - Herrero, A.
A1  - Ulaczyk, Krzysztof
T1  - The tarantula massive binary monitoring
BT  - II. First SB2 orbital and spectroscopic analysis for the Wolf-Rayet binary R145
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159 d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300 M-circle dot, making it a candidate for being the most massive star known to date. While the primary is a known late-type, H-rich Wolf-Rayet star (WN6h), the secondary has so far not been unambiguously detected. Using moderate-resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0 : 78 and minimum masses of M-1 sin(3) i approximate to M-2 sin(3) i = 13 +/- 2 M-circle dot, with q = M-2/M-1 = 1.01 +/- 0.07. An analysis of emission excess stemming from a wind-wind collision yields an inclination similar to that obtained from polarimetry (i = 39 +/- 6 degrees). Our analysis thus implies M-1 = 53(-20)(+40) and M2 = 54(-20)(+40) M-circle dot, excluding M-1 > 300 M-circle dot. A detailed comparison with evolution tracks calculated for single and binary stars together with the high eccentricity suggests that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of approximate to 80 M-circle dot and initial masses of M-i,M-1 approximate to 10(5) and M-i,M-2 approximate to 90 M-circle dot, consistent with the upper limits of our derived orbital masses, and would imply an age of approximate to 2.2 Myr.
KW  - binaries: spectroscopic
KW  - stars: Wolf-Rayet
KW  - stars: massive
KW  - Magellanic Clouds
KW  - stars: individual: R 145
KW  - stars: atmospheres
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201629621
SN  - 1432-0746
VL  - 598
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Fürst, F.
A1  - Kretschmar, P.
A1  - Oskinova, Lidia M.
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
A1  - Shenar, Tomer
A1  - Hamann, Wolf-Rainer
T1  - Coupling hydrodynamics with comoving frame radiative transfer
BT  - Stellar wind stratification in the high-mass X-ray binary Vela X-1
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: early-type
KW  - stars: atmospheres
KW  - stars: massive
KW  - X-rays: binaries
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201731575
SN  - 1432-0746
VL  - 610
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - THES
A1  - Novakovic-Marinkovic, Nina
T1  - Optical control of bubble domains and skyrmions in thin films
T1  - Optische Kontrolle von Blasendomänen und Skyrmionen in dünnen Schichten
N2  - Laser induced switching offers an attractive possibility to manipulate small magnetic domains for prospective memory and logic devices on ultrashort time scales. Moreover, optical control of magnetization without high applied magnetic fields allows manipulation of magnetic domains individually and locally, without expensive heat dissipation. One of the major challenges for developing novel optically controlled magnetic memory and logic devices is reliable formation and annihilation of non-volatile magnetic domains that can serve as memory bits in ambient conditions. Magnetic skyrmions, topologically nontrivial spin textures, have been studied intensively since their discovery due to their stability and scalability in potential spintronic devices. However, skyrmion formation and, especially, annihilation processes are still not completely understood and further investigation on such mechanisms are needed. The aim of this thesis is to contribute to better understanding of the physical processes behind the optical control of magnetism in thin films, with the goal of optimizing material parameters and methods for their potential use in next generation memory and logic devices.
First part of the thesis is dedicated to investigation of all-optical helicity-dependent switching (AO-HDS) as a method for magnetization manipulation. AO-HDS in Co/Pt multilayer and CoFeB alloys with and without the presence of Dzyaloshinskii-Moriya interaction (DMI), which is a type of exchange interaction, have been investigated by magnetic imaging using photo-emission electron microscopy (PEEM) in combination with X-ray magnetic circular dichroism (XMCD). The results show that in a narrow range of the laser fluence, circularly polarized laser light induces a drag on domain walls. This enables a local deterministic transformation of the magnetic domain pattern from stripes to bubbles in out-of-plane magnetized Co/Pt multilayers, only controlled by the helicity of ultrashort laser pulses. The temperature and characteristic fields at which the stripe-bubble transformation occurs has been calculated using theory for isolated magnetic bubbles, using as parameters experimentally determined average size of stripe domains and the magnetic layer thickness.
The second part of the work aims at purely optical formation and annihilation of magnetic skyrmions by a single laser pulse. The presence of a skyrmion phase in the investigated CoFeB alloys was first confirmed using a Kerr microscope. Then the helicity-dependent skyrmion manipulation was studied using AO-HDS at different laser fluences. It was found that formation or annihilation individual skyrmions using AO-HDS is possible, but not always reliable, as fluctuations in the laser fluence or position can easily overwrite the helicity-dependent effect of AO-HDS. However, the experimental results and magnetic simulations showed that the threshold values for the laser fluence for the formation and annihilation of skyrmions are different. A higher fluence is required for skyrmion formation, and existing skyrmions can be annihilated by pulses with a slightly lower fluence. This provides a further option for controlling formation and annihilation of skyrmions using the laser fluence. Micromagnetic simulations provide additional insights into the formation and annihilation mechanism.
The ability to manipulate the magnetic state of individual skyrmions is of fundamental importance for magnetic data storage technologies. Our results show for the first time that the optical formation and annihilation of skyrmions is possible without changing the external field. These results enable further investigations to optimise the magnetic layer to maximise the energy gap between the formation and annihilation barrier. As a result, unwanted switching due to small laser fluctuations can be avoided and fully deterministic optical switching can be achieved.
N2  - Laser induziertes Schalten bietet eine attraktive Möglichkeit zur Manipulation kleiner magnetischer Domänen für zukünftige Speicher- und Logikbauteile auf ultrakurzen Zeitskalen. Darüber hinaus ermöglicht die rein optische Kontrolle der Magnetisierung ohne hohe angelegte Magnetfelder eine individuelle und lokale Manipulation magnetischer Domänen ohne teure Wärmeverluste. Eine der größten Herausforderungen bei der Entwicklung neuartiger optisch kontrollierter magnetischer Speicher- und Logikbauteile ist das zuverlässige Schreiben stabiler magnetischer Domänen, die unter Umgebungsbedingungen als Speicherbits dienen können. Magnetische Skyrmionen, topologisch nichttriviale wirbelf¨ormige Spin-Texturen, wurden seit ihrer Entdeckung aufgrund ihrer Stabilität und Skalierbarkeit in potenziellen spintronischen Bauelementen intensiv untersucht. Allerdings sind die Prozesse der Skyrmionenbildung und vor allem der Skyrmionenvernichtung noch immer nicht vollständig verstanden, so dass weitere Untersuchungen zu diesen Mechanismen erforderlich sind. Ziel dieser Arbeit ist es, zu einem besseren Verständnis der physikalischen Prozesse beizutragen, die der optischen Kontrolle magnetischer Texturen in dünnen Filmen zugrunde liegen, mit dem Ziel, die Materialparameter und Methoden für ihren potenziellen Einsatz in Speicher- und Logikbauteilen der nächsten Generation zu optimieren.
Der erste Teil der Arbeit widmet sich der Untersuchung des rein optischen helizitätsabhängigen Schaltens (AO-HDS) als Methode zur Manipulation der Magnetisierung. AO-HDS wurde in Co/Pt-Multilayern und CoFeB-Legierungen mit und ohne DMI (Dzyaloshinskii-Moriya-Wechselwirkung), einer Art Austauschwechselwirkung, mittels magnetischer Bildgebung durch Photoemissions-Elektronenmikroskopie (PEEM) in Kombination mit magnetischem Röntgendichroismus (XMCD) untersucht. Die Ergebnisse zeigen, dass zirkular polarisiertes Licht in einem bestimmten Bereich der Laserfluenz einen Zug auf Domänenwände ausübt. Dies ermöglicht eine lokale deterministische Umwandlung des magnetischen Domänenmusters von Streifen zu Blasen Domänen in unmagnetisierten Co/Pt- Multilagen, die ausschließlich durch die Helizität der ultrakurzen Laserpulse gesteuert wird. Die Temperatur und die charakteristischen Felder, bei denen die Umwandlung von Streifen zu Blasen Domänen stattfindet, wurden mithilfe der Theorie für isolierte magnetische Blasen berechnet, wobei die experimentell ermittelte durchschnittliche Größe der Streifendom¨anen und die Dicke der magnetischen Schicht als Parameter verwendet wurden.
Der zweite Teil der Arbeit zielt darauf ab, optimale Parameter zu ermitteln, die für das rein optische Schreiben und Löschen von magnetischen Skyrmionen in einem einzigen Laser Puls erforderlich sind. Das Vorhandensein einer Skyrmion-Phase in dafür verwendeten CoFeB-Legierungen wurde zunächst mit Hilfe eines Kerr-Mikroskops bestätigt. Dann wurde die helizitätsabhängige Skyrmion-Manipulation mittels AOHDS für verschiedene Laserfluenzen untersucht. Es zeigte sich, dass eine Schreiben oder Löschen einzelner Skyrmionen mittels AO-HDS zwar möglich ist, aber nicht immer zuverlässig, da Fluktuationen in der Laserfluenz oder Position den helizitätsabhängigen Effekt von AO-HDS leicht überschreiben können. Die experimentellen Ergebnisse und magnetischen Simulationen zeigten allerdings, dass die Schwellenwerte für die Laserfluenz zur Bildung und Vernichtung von Skyrmionen unterschiedlich sind. Für die Skyrmionenbildung wird eine höhere Fluenz benötigt, und bereits existierende Skyrmionen können durch Laser Pulse mit etwas geringerer Fluenz gelöscht werden. Dadurch bietet sich eine weitere Möglichkeit das Schreiben und Löschen von Skyrmionen durch die Laserfluenz zu kontrollieren. Mikromagnetische Simulationen liefern zusätzliche Erkenntnisse über den Schreib- und Löschmechanismus.
Die Möglichkeit, den magnetischen Zustand einzelner Skyrmionen zu manipulieren, ist von grundlegender Bedeutung für magnetische Datenspeichertechnologien. Unsere Ergebnisse zeigen erstmalig, dass die optische Bildung und Vernichtung von Skyrmionen ohne Änderung des externen Feldes möglich ist. Diese Ergebnisse ermöglichen weitere Untersuchungen zur Optimierung der magnetischen Schicht, um die Energielücke zwischen Bildungs- und Vernichtungsbarriere zu maximieren. Dadurch kann unerwünschtes Schalten aufgrund kleiner Laserfluktuationen vermieden und vollständig deterministisches optisches Schalten erreicht werden.
KW  - Skyrmions
KW  - bubble domains
KW  - magnetism
KW  - Skyrmionen
KW  - Blasendomänen
KW  - Magnetismus
KW  - dünne Schichten
KW  - thin films
Y1  - 2024
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-647069
ER  - 
TY  - JOUR
A1  - Woodfield, Emma E.
A1  - Horne, Richard B.
A1  - Glauert, S. A.
A1  - Menietti, J. D.
A1  - Shprits, Yuri
A1  - Kurth, William S.
T1  - Formation of electron radiation belts at Saturn by Z-mode wave acceleration
JF  - Nature Communications
N2  - At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included.
Y1  - 2018
U6  - https://doi.org/10.1038/s41467-018-07549-4
SN  - 2041-1723
VL  - 9
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Cao, Xing
A1  - Ni, Binbin
A1  - Summers, Danny
A1  - Shprits, Yuri
A1  - Gu, Xudong
A1  - Fu, Song
A1  - Lou, Yuequn
A1  - Zhang, Yang
A1  - Ma, Xin
A1  - Zhang, Wenxun
A1  - Huang, He
A1  - Yi, Juan
T1  - Sensitivity of EMIC wave-driven scattering loss of ring current protons to wave normal angle distribution
JF  - Geophysical research letters
N2  - Electromagnetic ion cyclotron waves have long been recognized to play a crucial role in the dynamic loss of ring current protons. While the field-aligned propagation approximation of electromagnetic ion cyclotron waves was widely used to quantify the scattering loss of ring current protons, in this study, we find that the wave normal distribution strongly affects the pitch angle scattering efficiency of protons. Increase of peak normal angle or angular width can considerably reduce the scattering rates of <= 10 keV protons. For >10 keV protons, the field-aligned propagation approximation results in a pronounced underestimate of the scattering of intermediate equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics.
Y1  - 2019
U6  - https://doi.org/10.1029/2018GL081550
SN  - 0094-8276
SN  - 1944-8007
VL  - 46
IS  - 2
SP  - 590
EP  - 598
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Massa, Derck
A1  - Oskinova, Lidia M.
A1  - Prinja, Raman
A1  - Ignace, Richard
T1  - Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant xi Per
BT  - the Connection between X-Rays and Large-scale Wind Structure
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7. III(n) ((f)) star xi Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of xi Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N IV lambda 1718 and Si IV lambda 1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si IV lambda 1402, N IV lambda 1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180 degrees, on the surface of the star. We note that the presence and persistence of two spots separated by 180 degrees suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in xi Per.
KW  - stars: activity
KW  - stars: early-type
KW  - stars: winds, outflows
KW  - ultraviolet: stars
KW  - X-rays: stars
Y1  - 2019
U6  - https://doi.org/10.3847/1538-4357/ab0283
SN  - 0004-637X
SN  - 1538-4357
VL  - 873
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Oskinova, Lidia M.
A1  - Feldmeier, Achim
A1  - Hamann, Wolf-Rainer
T1  - High-resolution X-ray spectroscopy of bright O-type stars
JF  - Monthly notices of the Royal Astronomical Society
N2  - Archival X-ray spectra of the four prominent single, non-magnetic O stars zeta Pup, zeta Ori, xi Per and zeta Oph, obtained in high resolution with Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles provide information about the shocked, hot gas which emits the X-radiation, and about the bulk of comparably cool stellar wind material which partly absorbs this radiation. In this paper, we synthesize X-ray line profiles with a model of a clumpy stellar wind. We find that the geometrical shape of the wind inhomogeneities is important: better agreement with the observations can be achieved with radially compressed clumps than with spherical clumps. The parameters of the model, i.e. chemical abundances, stellar radius, mass-loss rate and terminal wind velocity, are taken from existing analyses of UV and optical spectra of the programme stars. On this basis, we also calculate the continuum-absorption coefficient of the cool-wind material, using the Potsdam Wolf-Rayet (POWR) model atmosphere code. The radial location of X-ray emitting gas is restricted from analysing the FIR line ratios of helium-like ions. The only remaining free parameter of our model is the typical distance between the clumps; here, we assume that at any point in the wind there is one clump passing by per one dynamical time-scale of the wind. The total emission in a model line is scaled to the observation. There is a good agreement between synthetic and observed line profiles. We conclude that the X-ray emission line profiles in O stars can be explained by hot plasma embedded in a cool wind which is highly clumped in the form of radially compressed shell fragments.
KW  - stars : individual : zeta Pup
KW  - stars : individual : zeta Ori
KW  - stars : individual : xi Per
KW  - stars : individual : zeta Oph
KW  - X-rays : stars
Y1  - 2006
U6  - https://doi.org/10.1111/j.1365-2966.2006.10858.x
SN  - 0035-8711
VL  - 372
SP  - 313
EP  - 326
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - GEN
A1  - Kubatova, Brankica
A1  - Hamann, Wolf-Rainer
A1  - Kubat, Jiri
A1  - Oskinova, Lidia M.
T1  - 3D Monte Carlo Radiative Transfer in Inhomogeneous Massive Star Winds
BT  - Application to Resonance Line Formation
T2  - Radiative signatures from the cosmos
N2  - Already for decades it has been known that the winds of massive stars are inhomogeneous (i.e. clumped). To properly model observed spectra of massive star winds it is necessary to incorporate the 3-D nature of clumping into radiative transfer calculations. In this paper we present our full 3-D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. We use a set of parameters to describe dense as well as the rarefied wind components. At the same time, we account for non-monotonic velocity fields. We show how the 3-D density and velocity wind inhomogeneities strongly affect the resonance line formation. We also show how wind clumping can solve the discrepancy between P v and H alpha mass-loss rate diagnostics.
Y1  - 2019
SN  - 978-1-58381-925-8
SN  - 1050-3390
VL  - 519
SP  - 209
EP  - 212
PB  - Astronomical soc pacific
CY  - San Fransisco
ER  - 
TY  - JOUR
A1  - Robrade, Jan
A1  - Oskinova, Lidia M.
A1  - Schmitt, J. H. M. M.
A1  - Leto, Paolo
A1  - Trigilio, C.
T1  - Outstanding X-ray emission from the stellar radio pulsar CU Virginis
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Among the intermediate-mass magnetic chemically peculiar (MCP) stars, CU Vir is one of the most intriguing objects. Its 100% circularly polarized beams of radio emission sweep the Earth as the star rotates, thereby making this strongly magnetic star the prototype of a class of nondegenerate stellar radio pulsars. While CU Vir is well studied in radio, its high-energy properties are not known. Yet, X-ray emission is expected from stellar magnetospheres and confined stellar winds. Aims. Using X-ray data we aim to test CU Vir for intrinsic X-ray emission and investigate mechanisms responsible for its generation. Methods. We present X-ray observations performed with XMM-Newton and Chandra and study obtained X-ray images, light curves, and spectra. Basic X-ray properties are derived from spectral modelling and are compared with model predictions. In this context we investigate potential thermal and nonthermal X-ray emission scenarios. Results. We detect an X-ray source at the position of CU Vir. With LX approximate to 3 x 10(28) erg s(-1) it is moderately X-ray bright, but the spectrum is extremely hard compared to other Ap stars. Spectral modelling requires multi-component models with predominant hot plasma at temperatures of about T-X = 25MK or, alternatively, a nonthermal spectral component. Both types of model provide a virtually equivalent description of the X-ray spectra. The Chandra observation was performed six years later than those by XMM-Newton, yet the source has similar X-ray flux and spectrum, suggesting a steady and persistent X-ray emission. This is further confirmed by the X-ray light curves that show only mild X-ray variability. Conclusions. CU Vir is also an exceptional star at X-ray energies. To explain its full X-ray properties, a generating mechanism beyond standard explanations, like the presence of a low-mass companion or magnetically confined wind-shocks, is required. Magnetospheric activity might be present or, as proposed for fast-rotating strongly magnetic Bp stars, the X-ray emission of CU Vir is predominantly auroral in nature.
KW  - individual: CU Vir
KW  - stars: activity
KW  - stars: chemically peculiar
KW  - stars: magnetic field
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833492
SN  - 1432-0746
VL  - 619
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Leto, Paolo
A1  - Trigilio, C.
A1  - Oskinova, Lidia M.
A1  - Ignace, R.
A1  - Buemi, C. S.
A1  - Umana, G.
A1  - Ingallinera, A.
A1  - Leone, Francesco
A1  - Phillips, N. M.
A1  - Agliozzo, Claudia
A1  - Todt, Helge Tobias
A1  - Cerrigone, L.
T1  - A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.
KW  - stars: chemically peculiar
KW  - stars: early-type
KW  - stars: individual: HR 5907
KW  - stars: magnetic field
KW  - radio continuum: stars
KW  - X-rays: stars
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty244
SN  - 0035-8711
SN  - 1365-2966
VL  - 476
IS  - 1
SP  - 562
EP  - 579
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lidia M.
A1  - Bulik, Tomasz
A1  - Gomez-Moran, Ada Nebot
T1  - Infrared outbursts as potential tracers of common-envelope events in high-mass X-ray binary formation
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Classic massive binary evolutionary scenarios predict that a transitional common-envelope (CE) phase could be preceded as well as succeeded by the evolutionary stage when a binary consists of a compact object and a massive star, that is, a high-mass X-ray binary (HMXB). The observational manifestations of common envelope are poorly constrained. We speculate that its ejection might be observed in some cases as a transient event at mid-infrared (IR) wavelengths. Aims. We estimate the expected numbers of CE ejection events and HMXBs per star formation unit rate, and compare these theoretical estimates with observations. Methods. We compiled a list of 85 mid-IR transients of uncertain nature detected by the Spitzer Infrared Intensive Transients Survey and searched for their associations with X-ray, optical, and UV sources. Results. Confirming our theoretical estimates, we find that only one potential HMXB may be plausibly associated with an IR-transient and tentatively propose that X-ray source NGC4490-X40 could be a precursor to the SPIRITS 16az event. Among other interesting sources, we suggest that the supernova remnant candidate [BWL2012] 063 might be associated with SPIRITS 16ajc. We also find that two SPIRITS events are likely associated with novae, and seven have potential optical counterparts. Conclusions. The massive binary evolutionary scenarios that involve CE events do not contradict currently available observations of IR transients and HMXBs in star-forming galaxies.
KW  - X-rays: binaries
KW  - stars: massive
KW  - infrared: general
KW  - infrared: galaxies
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201832925
SN  - 0004-6361
SN  - 1432-0746
VL  - 613
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - González-Galán, Ana
A1  - Oskinova, Lidia M.
A1  - Popov, Sergei B.
A1  - Haberl, F.
A1  - Kühnel, M.
A1  - Gallagher, John S.
A1  - Schurch, Matthew
A1  - Guerrero, Martín A.
T1  - A multiwavelength study of SXP 1062, the long-period X-ray pulsar associated with a supernova remnant
JF  - Monthly notices of the Royal Astronomical Society
N2  - SXP 1062 is a Be X-ray binary (BeXB) located in the Small Magellanic Cloud. It hosts a long-period X-ray pulsar and is likely associated with the supernova remnant MCSNR J0127−7332. In this work we present a multiwavelength view on SXP 1062 in different luminosity regimes. We consider monitoring campaigns in optical (OGLE survey) and X-ray (Swift telescope). During these campaigns a tight coincidence of X-ray and optical outbursts is observed. We interpret this as typical Type I outbursts as often detected in BeXBs at periastron passage of the neutron star (NS). To study different X-ray luminosity regimes in depth, during the source quiescence we observed it with XMM–Newton while Chandra observations followed an X-ray outburst. Nearly simultaneously with Chandra observations in X-rays, in optical the RSS/SALT telescope obtained spectra of SXP 1062. On the basis of our multiwavelength campaign we propose a simple scenario where the disc of the Be star is observed face-on, while the orbit of the NS is inclined with respect to the disc. According to the model of quasi-spherical settling accretion our estimation of the magnetic field of the pulsar in SXP 1062 does not require an extremely strong magnetic field at the present time.
KW  - stars: neutron
KW  - pulsars: individual: SXP 1062
KW  - galaxies: individual: Small Magellanic Cloud
KW  - X-rays: binaries
Y1  - 2017
U6  - https://doi.org/10.1093/mnras/stx3127
SN  - 0035-8711
SN  - 1365-2966
VL  - 475
IS  - 2
SP  - 2809
EP  - 2821
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Martinez-Chicharro, M.
A1  - Torrejon, J. M.
A1  - Oskinova, Lidia M.
A1  - Furst, F.
A1  - Postnov, K.
A1  - Rodes-Roca, J. J.
A1  - Hainich, Rainer
A1  - Bodaghee, A.
T1  - Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700-37
JF  - Monthly notices of the Royal Astronomical Society
N2  - Based on new Chandra X-ray telescope data, we present empirical evidence of plasma Compton cooling during a flare in the non-pulsating massive X-ray binary 4U1700-37. This behaviour might be explained by quasi-spherical accretion on to a slowly rotating magnetized neutron star (NS). In quiescence, the NS in 4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is supported by the detection of mHz quasi-periodic oscillations likely produced by its convection cells. The high plasma temperature and the relatively low X-ray luminosity observed during the quiescence, point to a small emitting area similar to 1 km, compatible with a hotspot on an NS surface. The sudden transition from a radiative to a significantly more efficient Compton cooling regime triggers an episode of enhanced accretion resulting in a flare. During the flare, the plasma temperature drops quickly. The predicted luminosity for such transitions, similar to 3 x 10(35) erg s(-1), is very close to the luminosity of 4U1700-37 during quiescence. The transition may be caused by the accretion of a clump in the stellar wind of the donor star. Thus, a magnetized NS nature of the compact object is strongly favoured.
KW  - stars: individual: 4U1700-37
KW  - V*V884 Sco
KW  - X-rays: binaries
Y1  - 2017
U6  - https://doi.org/10.1093/mnrasl/slx165
SN  - 0035-8711
SN  - 1365-2966
VL  - 473
IS  - 1
SP  - L74
EP  - L78
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Torrejon, J. M.
A1  - Reig, Pablo
A1  - Fürst, F.
A1  - Martinez-Chicharro, M.
A1  - Postnov, K.
A1  - Oskinova, Lidia M.
T1  - NuSTAR rules out a cyclotron line in the accreting magnetar candidate 4U2206+54
JF  - Monthly notices of the Royal Astronomical Society
N2  - Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high-mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to 5750 +/- 10 s, confirming the rapid spin-down rate (nu)over dot = -1.8 x 10(-14) Hz s(-1). This behaviour might be explained by the presence of a strongly magnetized neutron star (B-s > several times 10(13) G) accreting from the slow wind of its main-sequence O9.5 companion.
KW  - Stars: individual: 4U2206+54, BD+53 2790
KW  - X-rays: binaries
Y1  - 2018
U6  - https://doi.org/10.1093/mnras/sty1628
SN  - 0035-8711
SN  - 1365-2966
VL  - 479
IS  - 3
SP  - 3366
EP  - 3372
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Gruner, David
A1  - Hainich, Rainer
A1  - Sander, Andreas Alexander Christoph
A1  - Shenar, Tomer
A1  - Todt, Helge Tobias
A1  - Oskinova, Lidia M.
A1  - Ramachandran, Varsha
A1  - Ayres, T.
A1  - Hamann, Wolf-Rainer
T1  - The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system.
KW  - stars: individual: HD 93129A
KW  - stars: atmospheres
KW  - stars: fundamental parameters
KW  - stars: early-typeP
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833178
SN  - 1432-0746
VL  - 621
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Hainich, Rainer
A1  - Oskinova, Lidia M.
A1  - Shenar, Tomer
A1  - Marchant Campos, Pablo
A1  - Eldridge, J. J.
A1  - Sander, Andreas Alexander Christoph
A1  - Hamann, Wolf-Rainer
A1  - Langer, Norbert
A1  - Todt, Helge Tobias
T1  - Observational properties of massive black hole binary progenitors
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context:
The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models.
Aims:
Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate.
Methods:
Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities.
Results:
The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future.
Conclusions:
While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.
KW  - gravitational waves
KW  - binaries: close
KW  - stars: early-type
KW  - stars: atmospheres
KW  - stars: winds
KW  - outflows
KW  - stars: mass-loss
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201731449
SN  - 1432-0746
VL  - 609
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Fulmer, Leah M.
A1  - Gallagher, John S.
A1  - Hamann, Wolf-Rainer
A1  - Oskinova, Lidia M.
A1  - Ramachandran, Varsha
T1  - Testing massive star evolution, star-formation history, and feedback at low metallicity
BT  - photometric analysis of OB stars in the SMC Wing
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. 
The supergiant ionized shell SMC-SGS 1 (DEM 167), which is located in the outer Wing of the Small Magellanic Cloud (SMC), resembles structures that originate from an energetic star-formation event and later stimulate star formation as they expand into the ambient medium. However, stellar populations within and surrounding SMC-SGS 1 tell a different story. Aims. We present a photometric study of the stellar population encompassed by SMC-SGS 1 in order to trace the history of such a large structure and its potential influence on star formation within the low-density, low-metallicity environment of the SMC. 

Methods. 
For a stellar population that is physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV) photometry from the Galaxy Evolution Explorer with archival optical (V-band) photometry from the ESO Danish 1.54 m Telescope. Given their colors and luminosities, we estimated stellar ages and masses by matching observed photometry to theoretical stellar isochrone models. Results. We find that the investigated region supports an active, extended star-formation event spanning similar to 25-40 Myr ago, as well as continued star formation into the present. Using a standard initial mass function, we infer a lower bound on the stellar mass from this period of similar to 3 x 10(4) M-circle dot, corresponding to a star-formation intensity of similar to 6 x 10(-3) M-circle dot kpc(-2) yr(-1). 

Conclusions. 
The spatial and temporal distributions of young stars encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation over millions of years. Ongoing star formation, both along the edge and interior to SMC-SGS 1, suggests a combined stimulated and stochastic mode of star formation within the SMC Wing. We note that a slow expansion of the shell within this low-density environment may preserve molecular clouds within the volume of the shell, leaving them to form stars even after nearby stellar feedback expels local gas and dust.
KW  - galaxies
KW  - stellar content
KW  - stars
KW  - formation
KW  - individual
KW  - Small
KW  - Magellanic Cloud
Y1  - 2020
U6  - https://doi.org/10.1051/0004-6361/201834314
SN  - 0004-6361
SN  - 1432-0746
VL  - 633
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Toalá, Jesús Alberto
A1  - Bowman, Dominic
A1  - Van Reeth, Timothy
A1  - Todt, Helge Tobias
A1  - Dsilva, Karan
A1  - Shenar, Tomer
A1  - Koenigsberger, Gloria Suzanne
A1  - Estrada-Dorado, Sandino
A1  - Oskinova, Lidia M.
A1  - Hamann, Wolf-Rainer
T1  - Multiple variability time-scales of the early nitrogen-rich Wolf-Rayet star WR 7
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present the analysis of the optical variability of the early, nitrogen-rich Wolf-Rayet (WR) star WR 7. The analysis of multisector Transiting Exoplanet Survey Satellite (TESS) light curves and high-resolution spectroscopic observations confirm multiperiodic variability that is modulated on time-scales of years. We detect a dominant period of 2.6433 +/- 0.0005 d in the TESS sectors 33 and 34 light curves in addition to the previously reported high-frequency features from sector 7. We discuss the plausible mechanisms that may be responsible for such variability in WR 7, including pulsations, binarity, co-rotating interaction regions (CIRs), and clumpy winds. Given the lack of strong evidence for the presence of a stellar or compact companion, we suggest that WR 7 may pulsate in quasi-coherent modes in addition to wind variability likely caused by CIRs on top of stochastic low-frequency variability. WR 7 is certainly a worthy target for future monitoring in both spectroscopy and photometry to sample both the short (less than or similar to 1 d) and long (greater than or similar to 1000 d) variability time-scales.
KW  - stars: atmospheres
KW  - stars: evolution
KW  - stars: individual: WR 7
KW  - stars: winds
KW  - outflows
KW  - stars: Wolft-Rayet
Y1  - 2022
U6  - https://doi.org/10.1093/mnras/stac1455
SN  - 0035-8711
SN  - 1365-2966
VL  - 514
IS  - 2
SP  - 2269
EP  - 2277
PB  - Oxford University Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Oskinova, Lidia M.
A1  - Schaerer, Daniel
T1  - Ionization of He II in star-forming galaxies by X-rays from cluster winds and superbubbles
JF  - Astronomy and astrophysics : an international weekly journal
N2  - The nature of the sources powering nebular He II emission in star-forming galaxies remains debated, and various types of objects have been considered, including Wolf-Rayet stars, X-ray binaries, and Population III stars.
Modern X-ray observations show the ubiquitous presence of hot gas filling star-forming galaxies. We use a collisional ionization plasma code to compute the specific He II ionizing flux produced by hot gas and show that if its temperature is not too high (less than or similar to 2.5 MK), then the observed levels of soft diffuse X-ray radiation could explain He II ionization in galaxies.
To gain a physical understanding of this result, we propose a model that combines the hydrodynamics of cluster winds and hot superbubbles with observed populations of young massive clusters in galaxies. We find that in low-metallicity galaxies, the temperature of hot gas is lower and the production rate of He II ionizing photons is higher compared to high-metallicity galaxies. The reason is that the slower stellar winds of massive stars in lower-metallicity galaxies input less mechanical energy in the ambient medium. 
Furthermore, we show that ensembles of star clusters up to similar to 10-20 Myr old in galaxies can produce enough soft X-rays to induce nebular He II emission. We discuss observations of the template low-metallicity galaxy I Zw 18 and suggest that the He II nebula in this galaxy is powered by a hot superbubble. 
Finally, appreciating the complex nature of stellar feedback, we suggest that soft X-rays from hot superbubbles are among the dominant sources of He II ionizing flux in low-metallicity star-forming galaxies.
KW  - galaxies
KW  - ISM
KW  - high-redshift
KW  - bubbles
KW  - X-rays
Y1  - 2022
U6  - https://doi.org/10.1051/0004-6361/202142520
SN  - 0004-6361
SN  - 1432-0746
VL  - 661
PB  - EDP Sciences
CY  - Les Ulis
ER  -