FORMATION OF A DOUBLE-DECKER MAGNETIC FLUX ROPE IN THE SIGMOIDAL SOLAR ACTIVE REGION 11520

Abstract

In this paper, we address the formation of a magnetic flux rope (MFR) that\nerupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15.\nThrough analyzing the long-term evolution of the associated active region\nobserved by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic\nImager on board the Solar Dynamics Observatory, it is found that the twisted\nfield of an MFR, indicated by a continuous S-shaped sigmoid, is built up from\ntwo groups of sheared arcades near the main polarity inversion line half day\nbefore the eruption. The temperature within the twisted field and sheared\narcades is higher than that of the ambient volume, suggesting that magnetic\nreconnection most likely works there. The driver behind the reconnection is\nattributed to shearing and converging motions at magnetic footpoints with\nvelocities in the range of 0.1--0.6 km s$^{-1}$. The rotation of the preceding\nsunspot also contributes to the MFR buildup. Extrapolated three-dimensional\nnon-linear force-free field structures further reveal the locations of the\nreconnection to be in a bald-patch region and in a hyperbolic flux tube. About\ntwo hours before the eruption, indications for a second MFR in the form of an\nS-shaped hot channel are seen. It lies above the original MFR that continuously\nexists and includes a filament. The whole structure thus makes up a stable\ndouble-decker MFR system for hours prior to the eruption. Eventually, after\nentering the domain of instability, the high-lying MFR impulsively erupts to\ngenerate a fast coronal mass ejection and X-class flare; while the low-lying\nMFR remains behind and continuously maintains the sigmoidicity of the active\nregion.\n