@article{ToeroekLeakeTitovetal.2014, author = {Toeroek, T. and Leake, J. E. and Titov, Viacheslav S. and Archontis, V. and Mikic, Z. and Linton, M. G. and Dalmasse, K. and Aulanier, Guillaume and Kliem, Bernhard}, title = {Distribution of electric currents in solar active regions}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, volume = {782}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {2041-8205}, doi = {10.1088/2041-8205/782/1/L10}, pages = {6}, year = {2014}, language = {en} } @article{ToeroekKliemBergeretal.2014, author = {Toeroek, T. and Kliem, Bernhard and Berger, M. A. and Linton, M. G. and Demoulin, P. and van Driel-Gesztelyi, L.}, title = {The evolution of writhe in kink-unstable flux ropes and erupting filaments}, series = {Plasma physics and controlled fusion}, volume = {56}, journal = {Plasma physics and controlled fusion}, number = {6}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0741-3335}, doi = {10.1088/0741-3335/56/6/064012}, pages = {7}, year = {2014}, abstract = {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.}, language = {en} } @article{vanDrielGesztelyiBakerToeroeketal.2014, author = {van Driel-Gesztelyi, L. and Baker, Daniel N. and Toeroek, T. and Pariat, E. and Green, L. M. and Williams, D. R. and Carlyle, J. and Valori, G. and Demoulin, P. and Kliem, Bernhard and Long, D. M. and Matthews, S. A. and Malherbe, J. -M.}, title = {Coronal magnetic reconnection driven by CME expansion-the 2011 June 7 event}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {788}, 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/788/1/85}, pages = {12}, year = {2014}, abstract = {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.}, 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} } @article{ChengDingZhangetal.2014, author = {Cheng, X. and Ding, M. D. and Zhang, J. and Sun, X. D. and Guo, Y. and Wang, Yi-Ming and Kliem, Bernhard and Deng, Y. Y.}, title = {Formation of a double-decker magnetic flux rope in the sigmoidal solar active region 11520}, 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 = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/789/2/93}, pages = {12}, year = {2014}, abstract = {In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line a half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s(-1). The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About 2 hr before the eruption, indications of a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region.}, language = {en} }