Two-Stage Nature of a Solar Flare with Parallel and Semi-Circular Ribbons

TL;DR

This study analyzes a complex solar flare exhibiting both parallel and semi-circular ribbons, revealing a two-stage magnetic reconnection process involving standard flare reconnection and dome interaction, supported by multi-wavelength observations and magnetic field extrapolation.

Contribution

It provides a detailed analysis of a rare solar flare with both ribbon types occurring simultaneously, highlighting the interaction between different magnetic reconnection processes.

Findings

Two distinct brightening episodes in quasi-parallel ribbons correlate with HXR peaks.

Semi-circular ribbon brightening coincides with the local HXR minimum.

Magnetic field extrapolation reveals a dome-like structure influencing reconnection dynamics.

Abstract

Flare ribbons with parallel and circular morphologies are typically associated with different magnetic reconnection models, and the simultaneous observation of both types in a single event remains rare. Using multi-wavelength observations from a tandem of instruments, we present an M8.2-class flare that occurred on 2023 September 20, which produced quasi-parallel and semi-circular ribbons. The complex evolution of the flare includes two distinct brightening episodes in the quasi-parallel ribbons, corresponding to the two major peaks in the hard X-ray (HXR) light curve. In contrast, the brightening of semi-circular ribbons temporally coincides with the local minimum between the two peaks. Using potential field extrapolation, we reconstruct an incomplete dome-like magnetic structure with a negative polarity embedded within the northwestern part of the semi-circular positive polarity. Consequently, the magnetic configuration comprises two sets of field lines with distinct magnetic connectivities. We suggest that the standard flare reconnection accounts for the two-stage brightening of quasi-parallel ribbons associated with the two HXR peaks. Between the two stages, this process is constrained by the interaction of eruptive structures with the dome. The interaction drives the quasi-separatrix layer reconnection, leading to the brightening of semi-circular ribbons. It also suppresses the standard flare reconnection, resulting in a delayed second HXR peak.