TY - JOUR A1 - Kliem, Bernhard A1 - Toeroek, Tibor A1 - Titov, Viacheslav S. A1 - Lionello, Roberto A1 - Linker, Jon A. A1 - Liu, Rui A1 - Liu, Chang A1 - Wang, Haimin T1 - Slow rise and partial eruption of a double-decker filament. II. A double flux rope model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically becoming unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I. It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium. KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/792/2/107 SN - 0004-637X SN - 1538-4357 VL - 792 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Louis, Rohan E. A1 - Puschmann, Klaus G. A1 - Kliem, Bernhard A1 - Balthasar, Horst A1 - Denker, Carsten T1 - Sunspot splitting triggering an eruptive flare JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - Sun: flares KW - sunspots KW - Sun: photosphere KW - Sun: chromosphere KW - techniques: photometric Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201321106 SN - 0004-6361 SN - 1432-0746 VL - 562 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gao, Guan-Nan A1 - Wang, Min A1 - Lin, Jun A1 - Wu, Ning A1 - Tan, Cheng-Ming A1 - Kliem, Bernhard A1 - Su, Yang T1 - Radio observations of the fine structure inside a post-CME current sheet JF - Research in astronomy and astrophysics : a publication of the Chinese Astronomical Society and National Astronomical Observatories, Chinese Academy of Sciences N2 - A solar radio burst was observed in a coronal mass ejection/flare event by the Solar Broadband Radio Spectrometer at the Huairou Solar Observing Station on 2004 December 1. The data exhibited various patterns of plasma motions, suggestive of the interaction between sunward moving plasmoids and the flare loop system during the impulsive phase of the event. In addition to the radio data, the associated white-light, H alpha, extreme ultraviolet light, and soft and hard X-rays were also studied. KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares KW - Sun: solar radio bursts KW - Sun: magnetic reconnection Y1 - 2014 U6 - https://doi.org/10.1088/1674-4527/14/7/006 SN - 1674-4527 VL - 14 IS - 7 SP - 843 EP - 854 PB - Chinese Astronomical Society and National Astronomical Observatories, Chinese Academy of Sciences CY - Beijing 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 -