Structural evolution of a magnetic flux rope associated with a major flare in the solar active region 12205

Abstract

Solar eruptions are often generated as a result of the complex magnetic environment in solar active regions (ARs). Unravelling the relevant structure and evolution is vital to disclosing the underlying mechanisms that initiate such eruptions. In this work, we conduct a comprehensive study of the magnetic field structure and evolution responsible for a major flare eruption in a complex AR: NOAA 12205. The study is based on a detailed analysis of observations from the SDO and a time sequence of coronal magnetic field extrapolations. The AR is characterized by a long sequence of sunspots, harboring two groups of δ type that evolved dynamically via continual rotation, shearing, colliding, and flux cancellation. Our study suggests that the joint effect of the sunspot motions along a large-scale magnetic flux rope (MFR) supporting a filament was gradually built up along the main polarity inversion line. A quantitative analysis of the coronal magnetic evolution strongly indicates that an ideal instability of the MFR finally led to the major eruption of the X1.6 flare, although it was preceded by episodes of localized reconnections. These localized reconnections should play a key role in building up the unstable MFR by, for example, tether-cutting reconnection low near the photosphere, as driven by the shearing and flux cancellation. Through these reconnections, the MFR gains a significant amount of twisted flux and is lifted up to a height above the torus unstable threshold, at which the background restraining force decreases fast enough with the height.