How Can a Erupting Sigmoid Trigger Solar Flare with Parallel and Large–Scale Quasi–Circular Ribbons?

Abstract:

We present observations and analysis of an interesting sigmoid formation, eruption and the triggering of an solar flare with parallel and large-scale quasi-circular ribbons using SDO/AIA, SDO/HMI, GOES, RHESSI observations. We interpret the whole event in two different phases: preflare and main flare. The preflare phase was associated with the formation of sigmoid via tether-cutting type reconnection between the sheared field. The main phase also started with similar type reconnection between the sheared fields below the pre existing sigmoid structure at the circular polarity inversion line and the subsequent eruption of sigmoid. The multiwavelength observations as well as nonlinear force-free field extrapolation analysis show evidence of a large-scale fan-spine type magnetic configuration, with a sigmoid lying under a section of the fan dome. We are proposing the following scenario for the main event. The initial low lying tether-cutting type reconnection was believed to be responsible for the eruption of sigmoid. There after the reconnection occurred in the wake of the erupting sigmoid between the surrounding arcades, resulting in the formation of standard ribbons. In the next stage we believe the reconnection at the magnetic null point higher in the corona responsible for the formation of the extended large-scale quasi-circular ribbon.