Large Magnetic Flux Rope Formation in an X2.1 Flare Observed on 2011 September 6

Abstract Solar active region 11283 produced an X2.1 flare associated with a solar eruption on 2011 September 6. Observations revealed a preflare sigmoidal structure and a circular flare ribbon surrounding the typical two-ribbon structure, along with remote brightenings located at a considerable distance from the main flare site. To interpret these observations in terms of the dynamics of the three-dimensional coronal magnetic field, we conducted data-constrained magnetohydrodynamic simulations. Using a nonlinear force-free field as the initial condition, we reconstructed a realistic preflare magnetic environment, capturing a sheared sigmoid above the polarity inversion line surmounted by a fan–spine structure. Our simulations revealed that reconnection between the sigmoidal field, the adjacent fan–dome field lines, and the neighboring large loops facilitated the transfer of magnetic twist and led to the formation of a large magnetic flux rope (MFR). This transfer and propagation of twist are clearly visible throughout the MFR. As reconnection progresses, the entire fan–spine structure expands along with the evolving MFR. A notable outcome of the simulation is that the footpoints of the newly formed MFR align closely with the observed circular flare ribbon and the remote brightening region. Our findings suggest that a large MFR formed during the X2.1 flare, providing a coherent explanation for the observed phenomena.