TY  - JOUR
A1  - Cheng, Xin
A1  - Kliem, Bernhard
A1  - Ding, Mingde
T1  - Unambiguous evidence of filament splitting-induced partial eruptions
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - Coronal mass ejections are often considered to result from the full eruption of a magnetic flux rope (MFR). However, it is recognized that, in some events, the MFR may release only part of its flux, with the details of the implied splitting not completely established due to limitations in observations. Here, we investigate two partial eruption events including a confined and a successful one. Both partial eruptions are a consequence of the vertical splitting of a filament-hosting MFR involving internal reconnection. A loss of equilibrium in the rising part of the magnetic flux is suggested by the impulsive onset of both events and by the delayed onset of reconnection in the confined event. The remaining part of the flux might be line-tied to the photosphere in a bald patch (BP) separatrix surface, and we confirm the existence of extended BP sections for the successful eruption. The internal reconnection is signified by brightenings in the body of one filament and between the rising and remaining parts of both filaments. It evolves quickly into the standard current sheet reconnection in the wake of the eruption. As a result, regardless of being confined or successful, both eruptions produce hard X-ray sources and flare loops below the erupting but above the surviving flux, as well as a pair of flare ribbons enclosing the latter.
KW  - Sun: magnetic fields
KW  - Sun: corona
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: flares
Y1  - 2018
U6  - https://doi.org/10.3847/1538-4357/aab08d
SN  - 0004-637X
SN  - 1538-4357
VL  - 856
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Lee, Jeongwoo
A1  - White, Stephen M.
A1  - Liu, Chang
A1  - Kliem, Bernhard
A1  - Masuda, Satoshi
T1  - Magnetic Structure of a Composite Solar Microwave Burst
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - A composite flare consisting of an impulsive flare SOL2015-06-21T01:42 (GOES class M2.0) and a more gradual, long-duration flare SOL2015-06-21T02:36 (M2.6) from NOAA Active Region 12371, is studied using observations with the Nobeyama Radioheliograph (NoRH) and the Solar Dynamics Observatory (SDO). While composite flares are defined by their characteristic time profiles, in this paper we present imaging observations that demonstrate the spatial relationship of the two flares and allow us to address the nature of the evolution of a composite event. The NoRH maps show that the first flare is confined not only in time, but also in space, as evidenced by the stagnation of ribbon separation and the stationarity of the microwave source. The NoRH also detected another microwave source during the second flare, emerging from a different location where thermal plasma is so depleted that accelerated electrons could survive longer against Coulomb collisional loss. The AIA 131 angstrom images show that a sigmoidal EUV hot channel developed after the first flare and erupted before the second flare. We suggest that this eruption removed the high-lying flux to let the separatrix dome underneath reconnect with neighboring flux and the second microwave burst follow. This scenario explains how the first microwave burst is related to the much-delayed second microwave burst in this composite event.
KW  - Sun: activity
KW  - Sun: coronal mass ejections (CMEs)
KW  - Sun: flares
KW  - Sun: magnetic fields
KW  - Sun: radio radiation
KW  - Sun: UV radiation
Y1  - 2018
U6  - https://doi.org/10.3847/1538-4357/aaadbc
SN  - 0004-637X
SN  - 1538-4357
VL  - 856
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Verma, Meetu
A1  - Denker, Carsten
A1  - Balthasar, H.
A1  - Kuckein, Christoph
A1  - Rezaei, R.
A1  - Sobotka, Michal
A1  - Deng, N.
A1  - Wang, Haimin
A1  - Tritschler, A.
A1  - Collados, M.
A1  - Diercke, Andrea
A1  - González Manrique, Sergio Javier
T1  - High-resolution imaging and near-infrared spectroscopy of penumbral decay
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. Combining high-resolution spectropolarimetric and imaging data is key to understanding the decay process of sunspots as it allows us to scrutinize the velocity and magnetic fields of sunspots and their surroundings. Methods. Active region NOAA 12597 was observed on 2016 September 24 with the 1.5-meter GREGOR solar telescope using high-spatial-resolution imaging as well as imaging spectroscopy and near-infrared (NIR) spectropolarimetry. Horizontal proper motions were estimated with local correlation tracking, whereas line-of-sight (LOS) velocities were computed with spectral line fitting methods. The magnetic field properties were inferred with the "Stokes Inversions based on Response functions" (SIR) code for the Si I and Ca I NIR lines. Results. At the time of the GREGOR observations, the leading sunspot had two light bridges indicating the onset of its decay. One of the light bridges disappeared, and an elongated, dark umbral core at its edge appeared in a decaying penumbral sector facing the newly emerging flux. The flow and magnetic field properties of this penumbral sector exhibited weak Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap adjacent to the elongated umbral core and the penumbra in that penumbral sector displayed LOS velocities similar to granulation. The separating polarities of a new flux system interacted with the leading and central part of the already established active region. As a consequence, the leading spot rotated 55 degrees clockwise over 12 h. Conclusions. In the high-resolution observations of a decaying sunspot, the penumbral filaments facing the flux emergence site contained a darkened area resembling an umbral core filled with umbral dots. This umbral core had velocity and magnetic field properties similar to the sunspot umbra. This implies that the horizontal magnetic fields in the decaying penumbra became vertical as observed in flare-induced rapid penumbral decay, but on a very different time-scale.
KW  - Sun: photosphere
KW  - sunspots
KW  - Sun: magnetic fields
KW  - Sun: infrared
KW  - techniques: imaging spectroscopy
KW  - techniques: spectroscopic
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201731801
SN  - 1432-0746
VL  - 614
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Gömöry, Peter
A1  - Balthasar, Horst
A1  - Kuckein, Christoph
A1  - Koza, Julis
A1  - Veronig, Astrid M.
A1  - González Manrique, Sergio Javier
A1  - Kucera, Ales
A1  - Schwartz, Pavol
A1  - Hanslmeier, Arnold
T1  - Flare-induced changes of the photospheric magnetic field in a delta-spot deduced from ground-based observations
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. Changes of the magnetic field and the line-of-sight velocities in the photosphere are being reported for an M-class flare that originated at a delta-spot belonging to active region NOAA 11865. Methods. High-resolution ground-based near-infrared spectropolarimetric observations were acquired simultaneously in two photospheric spectral lines, Fe I 10783 angstrom and Si I 10786 angstrom, with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The observations covered several stages of the M-class flare. Inversions of the full-Stokes vector of both lines were carried out and the results were put into context using (extreme)-ultraviolet filtergrams from the Solar Dynamics Observatory (SDO). Results. The active region showed high flaring activity during the whole observing period. After the M-class flare, the longitudinal magnetic field did not show significant changes along the polarity inversion line (PIL). However, an enhancement of the transverse magnetic field of approximately 550G was found that bridges the PIL and connects umbrae of opposite polarities in the delta-spot. At the same time, a newly formed system of loops appeared co-spatially in the corona as seen in 171 angstrom filtergrams of the Atmospheric Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the magnetic connection between the umbrae already existed in the upper atmosphere before the M-class flare and became visible only later when it was filled with hot plasma. The photospheric Doppler velocities show a persistent upflow pattern along the PIL without significant changes due to the flare. Conclusions. The increase of the transverse component of the magnetic field after the flare together with the newly formed loop system in the corona support recent predictions of flare models and flare observations.
KW  - Sun: magnetic fields
KW  - sunspots
KW  - Sun: photosphere
KW  - Sun: flares
KW  - techniques: polarimetric
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201730644
SN  - 1432-0746
VL  - 602
SP  - 14
EP  - 27
PB  - EDP Sciences
CY  - Les Ulis
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  - Nishikawa, Ken-Ichi
A1  - Frederiksen, J. T.
A1  - Nordlund, A.
A1  - Mizuno, Y.
A1  - Hardee, P. E.
A1  - Niemiec, J.
A1  - Gomez, J. L.
A1  - Dutan, I.
A1  - Meli, A.
A1  - Sol, H.
A1  - Pohl, Martin
A1  - Hartmann, D. H.
T1  - EVOLUTION OF GLOBAL RELATIVISTIC JETS: COLLIMATIONS AND EXPANSION WITH
kKHI AND THE WEIBEL INSTABILITY
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - In the study of relativistic jets one of the key open questions is their interaction with the environment. Here. we study the initial evolution of both electron-proton (e(-) - p(+)) and electron-positron (e(+/-)) relativistic jets, focusing on their lateral interaction with ambient plasma. We follow the evolution of toroidal magnetic fields generated by both the kinetic Kelvin-Helmholtz and Mushroom instabilities. For an e(-) - p(+) jet, the induced magnetic field collimates the jet and electrons are perpendicularly accelerated. As the instabilities saturate and subsequently weaken, the magnetic polarity switches from clockwise to counterclockwise in the middle of the jet. For an e(+/-) jet, we find strong mixing of electrons and positrons with the ambient plasma, resulting in the creation of a bow shock. The merging of current filaments generates density inhomogeneities that. initiate a forward shock. Strong jet-ambient plasma mixing prevents a full development of the jet (on the scale studied), revealing evidence for both jet collimation and particle acceleration in the forming bow shock. Differences in the magnetic field structure generated by e(-) - p(+) and e(+/-) jets may contribute to the polarization properties of the observed emission in AGN jets and gamma-ray bursts.
KW  - acceleration of particles
KW  - plasmas
KW  - radiation mechanisms: non-thermal
KW  - relativistic processes
KW  - stars: jets
KW  - Sun: magnetic fields
Y1  - 2016
U6  - https://doi.org/10.3847/0004-637X/820/2/94
SN  - 0004-637X
SN  - 1538-4357
VL  - 820
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Denker, Carsten
A1  - Heibel, C.
A1  - Rendtel, J.
A1  - Arlt, K.
A1  - Balthasar, H.
A1  - Diercke, Andrea
A1  - Gonzalez Manrique, Sergio Javier
A1  - Hofmann, A.
A1  - Kuckein, Christoph
A1  - Önel, H.
A1  - Valliappan, Senthamizh Pavai
A1  - Staude, J.
A1  - Verma, Meetu
T1  - Solar physics at the Einstein Tower
JF  - Astronomische Nachrichten = Astronomical notes
KW  - history and philosophy of astronomy
KW  - Sun: photosphere
KW  - Sun: magnetic fields
KW  - techniques: spectroscopic
KW  - telescopes
Y1  - 2016
U6  - https://doi.org/10.1002/asna.201612442
SN  - 0004-6337
SN  - 1521-3994
VL  - 337
SP  - 1105
EP  - 1113
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Verma, Meetu
A1  - Denker, Carsten
A1  - Böhm, F.
A1  - Balthasar, H.
A1  - Fischer, C. E.
A1  - Kuckein, Christoph
A1  - Gonzalez, N. Bello
A1  - Berkefeld, T.
A1  - Collados Vera, M.
A1  - Diercke, Andrea
A1  - Feller, A.
A1  - Gonzalez Manrique, Sergio Javier
A1  - Hofmann, A.
A1  - Lagg, A.
A1  - Nicklas, H.
A1  - Orozco Suarez, D.
A1  - Pator Yabar, A.
A1  - Rezaei, R.
A1  - Schlichenmaier, R.
A1  - Schmidt, D.
A1  - Schmidt, W.
A1  - Sigwarth, M.
A1  - Sobotka, M.
A1  - Solanki, S. K.
A1  - Soltau, D.
A1  - Staude, J.
A1  - Strassmeier, Klaus G.
A1  - Volkmer, R.
A1  - von der Lühe, O.
A1  - Waldmann, T.
T1  - Flow and magnetic field properties in the trailing sunspots of active region NOAA 12396
JF  - Astronomische Nachrichten = Astronomical notes
N2  - Improved measurements of the photospheric and chromospheric three-dimensional magnetic and flow fields are crucial for a precise determination of the origin and evolution of active regions. We present an illustrative sample of multi-instrument data acquired during a two-week coordinated observing campaign in August 2015 involving, among others, the GREGOR solar telescope (imaging and near-infrared spectroscopy) and the space missions Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS). The observations focused on the trailing part of active region NOAA 12396 with complex polarity inversion lines and strong intrusions of opposite polarity flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV spectral profiles in the photospheric Si i.1082.7 nm line, the chromospheric He I lambda 1083.0 nm triplet, and the photospheric Ca I lambda 1083.9 nm line. Carefully calibrated GRIS scans of the active region provided maps of Doppler velocity and magnetic field at different atmospheric heights. We compare quick-look maps with those obtained with the " Stokes Inversions based on Response functions" (SIR) code, which furnishes deeper insight into the magnetic properties of the region. We find supporting evidence that newly emerging flux and intruding opposite polarity flux are hampering the formation of penumbrae, i.e., a penumbra fully surrounding a sunspot is only expected after cessation of flux emergence in proximity to the sunspots. (C) 2016 WILEY-VCH Verlag GmbH& Co.KGaA, Weinheim
KW  - Sun: magnetic fields
KW  - sunspots
KW  - methods: data analysis
KW  - techniques: polarimetric
KW  - techniques: spectroscopic
Y1  - 2016
U6  - https://doi.org/10.1002/asna.201612447
SN  - 0004-6337
SN  - 1521-3994
VL  - 337
SP  - 1090
EP  - 1098
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Verma, Meetu
A1  - Denker, Carsten
A1  - Balthasar, H.
A1  - Kuckein, Christoph
A1  - González Manrique, Sergio Javier
A1  - Sobotka, M.
A1  - Gonzalez, N. Bello
A1  - Hoch, S.
A1  - Diercke, Andrea
A1  - Kummerow, Philipp
A1  - Berkefeld, T.
A1  - Collados Vera, M.
A1  - Feller, A.
A1  - Hofmann, A.
A1  - Kneer, F.
A1  - Lagg, A.
A1  - Löhner-Böttcher, J.
A1  - Nicklas, H.
A1  - Pastor Yabar, A.
A1  - Schlichenmaier, R.
A1  - Schmidt, D.
A1  - Schmidt, W.
A1  - Schubert, M.
A1  - Sigwarth, M.
A1  - Solanki, S. K.
A1  - Soltau, D.
A1  - Staude, J.
A1  - Strassmeier, Klaus G.
A1  - Volkmer, R.
A1  - von der Lühe, O.
A1  - Waldmann, T.
T1  - Horizontal flow fields in and around a small active region The transition period between flux emergence and decay
JF  - Polymers
N2  - Context. The solar magnetic field is responsible for all aspects of solar activity. Thus, emergence of magnetic flux at the surface is the first manifestation of the ensuing solar activity. Aims. Combining high-resolution and synoptic observations aims to provide a comprehensive description of flux emergence at photospheric level and of the growth process that eventually leads to a mature active region. Methods. The small active region NOAA 12118 emerged on 2014 July 17 and was observed one day later with the 1.5-m GREGOR solar telescope on 2014 July 18. High-resolution time-series of blue continuum and G-band images acquired in the blue imaging channel (BIC) of the GREGOR Fabry-Perot Interferometer (GFPI) were complemented by synoptic line-of-sight magnetograms and continuum images obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Horizontal proper motions and horizontal plasma velocities were computed with local correlation tracking (LCT) and the differential affine velocity estimator (DAVE), respectively. Morphological image processing was employed to measure the photometric and magnetic area, magnetic flux, and the separation profile of the emerging flux region during its evolution. Results. The computed growth rates for photometric area, magnetic area, and magnetic flux are about twice as high as the respective decay rates. The space-time diagram using HMI magnetograms of five days provides a comprehensive view of growth and decay. It traces a leaf-like structure, which is determined by the initial separation of the two polarities, a rapid expansion phase, a time when the spread stalls, and a period when the region slowly shrinks again. The separation rate of 0.26 km s(-1) is highest in the initial stage, and it decreases when the separation comes to a halt. Horizontal plasma velocities computed at four evolutionary stages indicate a changing pattern of inflows. In LCT maps we find persistent flow patterns such as outward motions in the outer part of the two major pores, a diverging feature near the trailing pore marking the site of upwelling plasma and flux emergence, and low velocities in the interior of dark pores. We detected many elongated rapidly expanding granules between the two major polarities, with dimensions twice as large as the normal granules.
KW  - Sun: photosphere
KW  - Sun: magnetic fields
KW  - techniques: image processing
KW  - methods: data analysis
Y1  - 2016
U6  - https://doi.org/10.1051/0004-6361/201628380
SN  - 1432-0746
VL  - 596
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Martinez Gonzalez, M. J.
A1  - Pastor Yabar, A.
A1  - Lagg, A.
A1  - Asensio Ramos, A.
A1  - Collados Vera, M.
A1  - Solanki, S. K.
A1  - Balthasar, H.
A1  - Berkefeld, T.
A1  - Denker, Carsten
A1  - Doerr, H. P.
A1  - Feller, A.
A1  - Franz, M.
A1  - González Manrique, Sergio Javier
A1  - Hofmann, A.
A1  - Kneer, F.
A1  - Kuckein, Christoph
A1  - Louis, R.
A1  - von der Lühe, O.
A1  - Nicklas, H.
A1  - Orozco, D.
A1  - Rezaei, R.
A1  - Schlichenmaier, R.
A1  - Schmidt, D.
A1  - Schmidt, W.
A1  - Sigwarth, M.
A1  - Sobotka, M.
A1  - Soltau, D.
A1  - Staude, J.
A1  - Strassmeier, Klaus G.
A1  - Verma, Meetu
A1  - Waldman, T.
A1  - Volkmer, R.
T1  - Inference of magnetic fields in the very quiet Sun
JF  - Journal of geophysical research : Earth surface
N2  - Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution. Aims. We present high-precision spectro-polarimetric data with high spatial resolution (0.4") of the very quiet Sun at 1.56 mu m obtained with the GREGOR telescope to shed some light on this complex magnetism. Methods. We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component. Results. Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak (similar to 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area similar to 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised.
KW  - Sun: atmosphere
KW  - Sun: magnetic fields
KW  - techniques: polarimetric
KW  - methods: observational
Y1  - 2016
U6  - https://doi.org/10.1051/0004-6361/201628449
SN  - 1432-0746
VL  - 596
PB  - EDP Sciences
CY  - Les Ulis
ER  -