TY  - JOUR
A1  - Veronig, Astrid M.
A1  - Podladchikova, Tatiana
A1  - Dissauer, Karin
A1  - Temmer, Manuela
A1  - Seaton, Daniel B.
A1  - Long, David
A1  - Guo, Jingnan
A1  - Vrsnak, Bojan
A1  - Harra, Louise
A1  - Kliem, Bernhard
T1  - Genesis and Impulsive Evolution of the 2017 September 10 Coronal Mass Ejection
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The X8.2 event of 2017 September 10 provides unique observations to study the genesis, magnetic morphology, and impulsive dynamics of a very fast coronal mass ejection (CME). Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot (T approximate to 10-15 MK) bright rim around a quickly expanding cavity, embedded inside a much larger CME shell (T approximate to 1-2 MK). The CME shell develops from a dense set of large AR loops ( greater than or similar to 0.5R(s)) and seamlessly evolves into the CME front observed in LASCO C2. The strong lateral overexpansion of the CME shell acts as a piston initiating the fast EUV wave. The hot cavity rim is demonstrated to be a manifestation of the dominantly poloidal flux and frozen-in plasma added to the rising flux rope by magnetic reconnection in the current sheet beneath. The same structure is later observed as the core of the white-light CME, challenging the traditional interpretation of the CME three-part morphology. The large amount of added magnetic flux suggested by these observations explains the extreme accelerations of the radial and lateral expansion of the CME shell and cavity, all reaching values of 5-10 km s(-2). The acceleration peaks occur simultaneously with the first RHESSI 100-300 keV hard X-ray burst of the associated flare, further underlining the importance of the reconnection process for the impulsive CME evolution. Finally, the much higher radial propagation speed of the flux rope in relation to the CME shell causes a distinct deformation of the white-light CME front and shock.
KW  - Sun: activity
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: flares
Y1  - 2018
U6  - https://doi.org/10.3847/1538-4357/aaeac5
SN  - 0004-637X
SN  - 1538-4357
VL  - 868
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - van Driel-Gesztelyi, L.
A1  - Baker, Daniel N.
A1  - Toeroek, T.
A1  - Pariat, E.
A1  - Green, L. M.
A1  - Williams, D. R.
A1  - Carlyle, J.
A1  - Valori, G.
A1  - Demoulin, Pascal
A1  - Kliem, Bernhard
A1  - Long, D. M.
A1  - Matthews, S. A.
A1  - Malherbe, J. -M.
T1  - Coronal magnetic reconnection driven by CME expansion-the 2011 June 7  event
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - Coronal mass ejections (CMEs) erupt and expand in a magnetically structured solar corona. Various indirect observational pieces of evidence have shown that the magnetic field of CMEs reconnects with surrounding magnetic fields, forming, e.g., dimming regions distant from the CME source regions. Analyzing Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on 2011 June 7, we present the first direct evidence of coronal magnetic reconnection between the fields of two adjacent active regions during a CME. The observations are presented jointly with a data-constrained numerical simulation, demonstrating the formation/intensification of current sheets along a hyperbolic flux tube at the interface between the CME and the neighboring AR 11227. Reconnection resulted in the formation of new magnetic connections between the erupting magnetic structure from AR 11226 and the neighboring active region AR 11227 about 200 Mm from the eruption site. The onset of reconnection first becomes apparent in the SDO/AIA images when filament plasma, originally contained within the erupting flux rope, is redirected toward remote areas in AR 11227, tracing the change of large-scale magnetic connectivity. The location of the coronal reconnection region becomes bright and directly observable at SDO/AIA wavelengths, owing to the presence of down-flowing cool, dense (1010 cm(-3)) filament plasma in its vicinity. The high-density plasma around the reconnection region is heated to coronal temperatures, presumably by slow-mode shocks and Coulomb collisions. These results provide the first direct observational evidence that CMEs reconnect with surrounding magnetic structures, leading to a large-scale reconfiguration of the coronal magnetic field.
KW  - magnetic reconnection
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: magnetic fields
KW  - Sun: UV radiation
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/788/1/85
SN  - 0004-637X
SN  - 1538-4357
VL  - 788
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Valori, Gherarod
A1  - Demoulin, Pascal
A1  - Pariat, E.
A1  - Masson, S.
T1  - Accuracy of magnetic energy computations
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. For magnetically driven events, the magnetic energy of the system is the prime energy reservoir that fuels the dynamical evolution. In the solar context, the free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. A trustworthy estimation of the magnetic energy is therefore needed in three-dimensional (3D) models of the solar atmosphere, e. g., in coronal fields reconstructions or numerical simulations.
 Aims. The expression of the energy of a system as the sum of its potential energy and its free energy (Thomson's theorem) is strictly valid when the magnetic field is exactly solenoidal. For numerical realizations on a discrete grid, this property may be only approximately fulfilled. We show that the imperfect solenoidality induces terms in the energy that can lead to misinterpreting the amount of free energy present in a magnetic configuration.
 Methods. We consider a decomposition of the energy in solenoidal and nonsolenoidal parts which allows the unambiguous estimation of the nonsolenoidal contribution to the energy. We apply this decomposition to six typical cases broadly used in solar physics. We quantify to what extent the Thomson theorem is not satisfied when approximately solenoidal fields are used.
 Results. The quantified errors on energy vary from negligible to significant errors, depending on the extent of the nonsolenoidal component of the field. We identify the main source of errors and analyze the implications of adding a variable amount of divergence to various solenoidal fields. Finally, we present pathological unphysical situations where the estimated free energy would appear to be negative, as found in some previous works, and we identify the source of this error to be the presence of a finite divergence.
 Conclusions. We provide a method of quantifying the effect of a finite divergence in numerical fields, together with detailed diagnostics of its sources. We also compare the efficiency of two divergence-cleaning techniques. These results are applicable to a broad range of numerical realizations of magnetic fields.
KW  - magnetic fields
KW  - methods: numerical
KW  - Sun: surface magnetism
KW  - Sun: corona
Y1  - 2013
U6  - https://doi.org/10.1051/0004-6361/201220982
SN  - 0004-6361
VL  - 553
IS  - 2
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Toeroek, T.
A1  - Leake, J. E.
A1  - Titov, Viacheslav S.
A1  - Archontis, V.
A1  - Mikic, Z.
A1  - Linton, M. G.
A1  - Dalmasse, K.
A1  - Aulanier, Guillaume
A1  - Kliem, Bernhard
T1  - Distribution of electric currents in solar active regions
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
Y1  - 2014
U6  - https://doi.org/10.1088/2041-8205/782/1/L10
SN  - 2041-8205
SN  - 2041-8213
VL  - 782
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Toeroek, T.
A1  - Kliem, Bernhard
A1  - Berger, M. A.
A1  - Linton, M. G.
A1  - Demoulin, Pascal
A1  - van Driel-Gesztelyi, L.
T1  - The evolution of writhe in kink-unstable flux ropes and erupting filaments
JF  - Plasma physics and controlled fusion
N2  - The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-emptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold-Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of similar to 6 pi for the majority of filaments prior to their eruption.
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: filaments
Y1  - 2014
U6  - https://doi.org/10.1088/0741-3335/56/6/064012
SN  - 0741-3335
SN  - 1361-6587
VL  - 56
IS  - 6
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Liu, Rui
A1  - Kliem, Bernhard
A1  - Titov, Viacheslav S.
A1  - Chen, Jun
A1  - Wang, Yuming
A1  - Wang, Haimin
A1  - Liu, Chang
A1  - Xu, Yan
A1  - Wiegelmann, Thomas
T1  - STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We investigate the evolution of NOAA Active Region (AR) 11817 during 2013 August 10–12, when it developed a complex field configuration and produced four confined, followed by two eruptive, flares. These C-and-above flares are all associated with a magnetic flux rope (MFR) located along the major polarity inversion line, where shearing and converging photospheric flows are present. Aided by the nonlinear force-free field modeling, we identify the MFR through mapping magnetic connectivities and computing the twist number ${{ \mathcal T }}_{w}$ for each individual field line. The MFR is moderately twisted ($| {{ \mathcal T }}_{w}| \lt 2$) and has a well-defined boundary of high squashing factor Q. We found that the field line with the extremum $| {{ \mathcal T }}_{w}| $ is a reliable proxy of the rope axis, and that the MFR's peak $| {{ \mathcal T }}_{w}| $ temporarily increases within half an hour before each flare while it decreases after the flare peak for both confined and eruptive flares. This pre-flare increase in $| {{ \mathcal T }}_{w}| $ has little effect on the AR's free magnetic energy or any other parameters derived for the whole region, due to its moderate amount and the MFR's relatively small volume, while its decrease after flares is clearly associated with the stepwise decrease in the whole region's free magnetic energy due to the flare. We suggest that ${{ \mathcal T }}_{w}$ may serve as a useful parameter in forewarning the onset of eruption, and therefore, the consequent space weather effects. The helical kink instability is identified as the prime candidate onset mechanism for the considered flares.
KW  - coronal mass ejections (CMEs)
KW  - Sun: corona
KW  - Sun: filaments, pominences
KW  - Sun: flares
KW  - Sun: magnetic fields
Y1  - 2016
U6  - https://doi.org/10.3847/0004-637X/818/2/148
SN  - 0004-637X
SN  - 1538-4357
VL  - 818
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Kuckein, Christoph
A1  - Diercke, Andrea
A1  - González Manrique, Sergio Javier
A1  - Verma, Meetu
A1  - Loehner-Boettcher, Johannes
A1  - Socas-Navarro, H.
A1  - Balthasar, Horst
A1  - Sobotka, M.
A1  - Denker, Carsten
T1  - Ca II 8542 angstrom brightenings induced by a solar microflare
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. We study small-scale brightenings in Ca II 8542 angstrom line-core images to determine their nature and effect on localized heating and mass transfer in active regions. Methods. High-resolution two-dimensional spectroscopic observations of a solar active region in the near-infrared Ca II 8542 angstrom line were acquired with the GREGOR Fabry-Perot Interferometer attached to the 1.5-m GREGOR telescope. Inversions of the spectra were carried out using the NICOLE code to infer temperatures and line-of-sight (LOS) velocities. Response functions of the Ca II line were computed for temperature and LOS velocity variations. Filtergrams of the Atmospheric Imaging Assembly (AIA) and magnetograms of the Helioseismic and Magnetic Imager (HMI) were coaligned to match the ground-based observations and to follow the Ca II brightenings along all available layers of the atmosphere. Results. We identified three brightenings of sizes up to 2 ' x 2 ' that appeared in the Ca II 8542 angstrom line-core images. Their lifetimes were at least 1.5 min. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or Interface Region Imaging Spectrograph (IRIS) bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels; and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of similar to 20 ' next to the brightenings. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km s(-1). The structure did not disappear during the observations. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF.
KW  - Sun: photosphere
KW  - Sun: chromosphere
KW  - Sun: corona
KW  - Sun: activity
KW  - techniques: imaging spectroscopy
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201731319
SN  - 1432-0746
VL  - 608
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Kliem, Bernhard
A1  - Su, Y. N.
A1  - van Ballegooijen, A. A.
A1  - DeLuca, E. E.
T1  - Magnetohydrodynamic modeling of the solar eruption on 2010 APRIL 8
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The structure of the coronal magnetic field prior to eruptive processes and the conditions for the onset of eruption are important issues that can be addressed through studying the magnetohydrodynamic (MHD) stability and evolution of nonlinear force-free field (NLFFF) models. This paper uses data-constrained NLFFF models of a solar active region (AR) that erupted on 2010 April 8 as initial conditions in MHD simulations. These models, constructed with the techniques of flux rope insertion and magnetofrictional relaxation (MFR), include a stable, an approximately marginally stable, and an unstable configuration. The simulations confirm previous related results of MFR runs, particularly that stable flux rope equilibria represent key features of the observed pre-eruption coronal structure very well, and that there is a limiting value of the axial flux in the rope for the existence of stable NLFFF equilibria. The specific limiting value is located within a tighter range, due to the sharper discrimination between stability and instability by the MHD description. The MHD treatment of the eruptive configuration yields a very good agreement with a number of observed features, like the strongly inclined initial rise path and the close temporal association between the coronal mass ejection and the onset of flare reconnection. Minor differences occur in the velocity of flare ribbon expansion and in the further evolution of the inclination; these can be eliminated through refined simulations. We suggest that the slingshot effect of horizontally bent flux in the source region of eruptions can contribute significantly to the inclination of the rise direction. Finally, we demonstrate that the onset criterion, formulated in terms of a threshold value for the axial flux in the rope, corresponds very well to the threshold of the torus instability in the considered AR.
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: filaments, prominences
KW  - Sun: flares
KW  - Sun: magnetic fields
Y1  - 2013
U6  - https://doi.org/10.1088/0004-637X/779/2/129
SN  - 0004-637X
SN  - 1538-4357
VL  - 779
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Kliem, Bernhard
A1  - Lin, J.
A1  - Forbes, T. G.
A1  - Priest, E. R.
A1  - Toeroek, T.
T1  - Catastrophe versus instability for the eruption of a toroadal solar magnetic flux
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The onset of a solar eruption is formulated here as either a magnetic catastrophe or as an instability. Both start with the same equation of force balance governing the underlying equilibria. Using a toroidal flux rope in an external bipolar or quadrupolar field as a model for the current-carrying flux, we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for several representative evolutionary sequences in the stable domain of parameter space. We verify that this catastrophe and the torus instability occur at the same point; they are thus equivalent descriptions for the onset condition of solar eruptions.
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: filaments, prominences
KW  - Sun: flares
KW  - Sun: magnetic fields
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/789/1/46
SN  - 0004-637X
SN  - 1538-4357
VL  - 789
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Hassanin, Alshaimaa
A1  - Kliem, Bernhard
A1  - Seehafer, Norbert
T1  - Helical kink instability in the confined solar eruption on 2002 May 27
JF  - Astronomische Nachrichten = Astronomical notes
KW  - instabilities
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: flares
Y1  - 2016
U6  - https://doi.org/10.1002/asna.201612446
SN  - 0004-6337
SN  - 1521-3994
VL  - 337
SP  - 1082
EP  - 1089
PB  - Wiley-VCH
CY  - Weinheim
ER  -