The Role of Erupting Sigmoid in Triggering a Flare with Parallel and Large-Scale Quasi-Circular Ribbons

TL;DR

This study analyzes a solar flare event involving sigmoid eruption and complex ribbon formations, proposing a multi-stage reconnection mechanism within a fan-spine magnetic configuration, supported by multi-wavelength observations and magnetic field extrapolations.

Contribution

It introduces a novel multi-stage reconnection scenario explaining the formation of parallel and quasi-circular ribbons triggered by sigmoid eruption in a fan-spine magnetic structure.

Findings

Sigmoid formation via tether-cutting reconnection.

Eruption of sigmoid leads to parallel ribbons.

Null-type reconnection produces quasi-circular ribbon.

Abstract

In this paper, we present observations and analysis of an interesting sigmoid formation, eruption and the associated flare that occurred on 2014 April 18 using multi-wavelength data sets. We discuss the possible role of the sigmoid eruption in triggering the flare, which consists of two different set of ribbons: parallel ribbons as well as a large-scale quasi-circular ribbon. Several observational evidence and nonlinear force-free field extrapolation results show the existence of a large-scale fan-spine type magnetic configuration with a sigmoid lying under a section of the fan dome. The event can be explained with the following two phases. During the pre-flare phase, we observed the formation and appearance of sigmoid via tether-cutting reconnection between the two sets of sheared fields under the fan dome. The second, main flare phase, features the eruption of the sigmoid, the subsequent flare with parallel ribbons, and a quasi-circular ribbon. We propose the following multi-stage successive reconnections scenario for the main flare. First, tether-cutting reconnection is responsible for the formation and the eruption of the sigmoid structure. Second, the reconnection occurred in the wake of the erupting sigmoid produces the parallel flare ribbons on the both sides of the circular polarity inversion line. Third, the null-type reconnection higher in the corona, possibly triggered by the erupting sigmoid, leads to the formation of a large quasi-circular ribbon. For the first time we suggest a mechanism for this type of flare consisting of a double set of ribbons triggered by an erupting sigmoid in a large scale fan-spine type magnetic configuration.