@article{ChengKliemDing2018,
  author    = {Cheng, Xin and Kliem, Bernhard and Ding, Mingde},
  title     = {Unambiguous evidence of filament splitting-induced partial eruptions},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {856},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aab08d},
  pages     = {15},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{LeeWhiteLiuetal.2018,
  author    = {Lee, Jeongwoo and White, Stephen M. and Liu, Chang and Kliem, Bernhard and Masuda, Satoshi},
  title     = {Magnetic Structure of a Composite Solar Microwave Burst},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {856},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aaadbc},
  pages     = {10},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{VeronigPodladchikovaDissaueretal.2018,
  author    = {Veronig, Astrid M. and Podladchikova, Tatiana and Dissauer, Karin and Temmer, Manuela and Seaton, Daniel B. and Long, David and Guo, Jingnan and Vrsnak, Bojan and Harra, Louise and Kliem, Bernhard},
  title     = {Genesis and Impulsive Evolution of the 2017 September 10 Coronal Mass Ejection},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {868},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aaeac5},
  pages     = {17},
  year      = {2018},
  abstract  = {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.},
  language  = {en}
}
@article{GoemoeryBalthasarKuckeinetal.2017,
  author    = {G{\"o}m{\"o}ry, Peter and Balthasar, Horst and Kuckein, Christoph and Koza, Julis and Veronig, Astrid M. and Gonz{\´a}lez Manrique, Sergio Javier and Kucera, Ales and Schwartz, Pavol and Hanslmeier, Arnold},
  title     = {Flare-induced changes of the photospheric magnetic field in a delta-spot deduced from ground-based observations},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {602},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201730644},
  pages     = {14 -- 27},
  year      = {2017},
  abstract  = {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.},
  language  = {en}
}
@article{LiuKliemTitovetal.2016,
  author    = {Liu, Rui and Kliem, Bernhard and Titov, Viacheslav S. and Chen, Jun and Wang, Yuming and Wang, Haimin and Liu, Chang and Xu, Yan and Wiegelmann, Thomas},
  title     = {STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {818},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/0004-637X/818/2/148},
  pages     = {22},
  year      = {2016},
  abstract  = {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.},
  language  = {en}
}
@article{HassaninKliemSeehafer2016,
  author    = {Hassanin, Alshaimaa and Kliem, Bernhard and Seehafer, Norbert},
  title     = {Helical kink instability in the confined solar eruption on 2002 May 27},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {337},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201612446},
  pages     = {1082 -- 1089},
  year      = {2016},
  language  = {en}
}
@misc{SuKliemvanBallegooijenetal.2012,
  author    = {Su, Yingna and Kliem, Bernhard and van Ballegooijen, Adriaan and Deluca, Edward},
  title     = {Numerical simulations of the CME on 2010 April 8},
  series = {Solar and Astrophysical Dynamos and Magnetic Activity},
  journal   = {Solar and Astrophysical Dynamos and Magnetic Activity},
  number    = {674},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41488},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-414887},
  pages     = {575 -- 576},
  year      = {2012},
  abstract  = {We present 3D zero-beta ideal MHD simulations of the solar flare/CME event that occurred in Active Region 11060 on 2010 April 8. The initial magnetic configurations of the two simulations are stable nonlinear force-free field and unstable magnetic field models constructed by Su et al. (2011) using the flux rope insertion method. The MHD simulations confirm that the stable model relaxes to a stable equilibrium, while the unstable model erupts as a CME. Comparisons between observations and MHD simulations of the CME are also presented.},
  language  = {en}
}
@article{DalmasseAulanierDemoulinetal.2015,
  author    = {Dalmasse, Kevin and Aulanier, Guillaume and Demoulin, P. and Kliem, Bernhard and T{\"o}r{\"o}k, Tibor and Pariat, E.},
  title     = {The origin of net electric currents in solar active regions},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {810},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/810/1/17},
  pages     = {14},
  year      = {2015},
  abstract  = {There is a recurring question in solar physics regarding whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Other sources of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net versus neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both direct and return currents, (2) induce very weak compression currents-not observed in 2.5D-in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line-a special condition that is rarely observed. We conclude that. photospheric flows,. as magnetic flux emergence, can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields that possess a net coronal current.},
  language  = {en}
}
@article{LouisPuschmannKliemetal.2014,
  author    = {Louis, Rohan E. and Puschmann, Klaus G. and Kliem, Bernhard and Balthasar, Horst and Denker, Carsten},
  title     = {Sunspot splitting triggering an eruptive flare},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {562},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201321106},
  pages     = {15},
  year      = {2014},
  abstract  = {Aims. We investigate how the splitting of the leading sunspot and associated flux emergence and cancellation in active region NOAA 11515 caused an eruptive M5.6 flare on 2012 July 2. Methods. Continuum intensity, line-of-sight magnetogram, and dopplergram data of the Helioseismic and Magnetic Imager were employed to analyse the photospheric evolution. Filtergrams in H alpha and He I 10830 angstrom of the Chromospheric Telescope at the Observatorio del Teide, Tenerife, track the evolution of the flare. The corresponding coronal conditions were derived from 171 angstrom and 304 angstrom images of the Atmospheric Imaging Assembly. Local correlation tracking was utilized to determine shear flows. Results. Emerging flux formed a neutral line ahead of the leading sunspot and new satellite spots. The sunspot splitting caused a long-lasting flow towards this neutral line, where a filament formed. Further flux emergence, partly of mixed polarity, as well as episodes of flux cancellation occurred repeatedly at the neutral line. Following a nearby C-class precursor flare with signs of interaction with the filament, the filament erupted nearly simultaneously with the onset of the M5.6 flare and evolved into a coronal mass ejection. The sunspot stretched without forming a light bridge, splitting unusually fast (within about a day, complete approximate to 6 h after the eruption) in two nearly equal parts. The front part separated strongly from the active region to approach the neighbouring active region where all its coronal magnetic connections were rooted. It also rotated rapidly (by 4.9 degrees h(-1)) and caused significant shear flows at its edge. Conclusions. The eruption resulted from a complex sequence of processes in the (sub-)photosphere and corona. The persistent flows towards the neutral line likely caused the formation of a flux rope that held the filament. These flows, their associated flux cancellation, the emerging flux, and the precursor flare all contributed to the destabilization of the flux rope. We interpret the sunspot splitting as the separation of two flux bundles differently rooted in the convection zone and only temporarily joined in the spot. This explains the rotation as the continued rise of the separating flux, and it implies that at least this part of the sunspot was still connected to its roots deep in the convection zone.},
  language  = {en}
}
@article{KliemLinForbesetal.2014,
  author    = {Kliem, Bernhard and Lin, J. and Forbes, T. G. and Priest, E. R. and Toeroek, T.},
  title     = {Catastrophe versus instability for the eruption of a toroadal solar magnetic flux},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {789},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/789/1/46},
  pages     = {13},
  year      = {2014},
  abstract  = {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.},
  language  = {en}
}