Two-stage energy release process of a confined flare with double HXR peaks

TL;DR

This paper investigates a confined solar flare with a unique double-peaked X-ray profile, revealing a two-stage energy release process involving loop-loop reconnection and multi-thermal plasma components.

Contribution

It provides detailed multi-wavelength observations and analysis of a confined flare with double peaks, highlighting a novel two-stage energy release mechanism.

Findings

First peak is non-thermal dominated.

Second peak involves hot and super-hot thermal components.

Double peaks are separated by 4 minutes with distinct spectral features.

Abstract

A complete understanding of the onset and subsequent evolution of confined flares has not been achieved. Earlier studies mainly analyzed disk events so as to reveal their magnetic topology and cause of confinement. In this study, taking advantage of a tandem of instruments working at different wavelengths of X-rays, EUVs, and microwaves, we present dynamic details of a confined flare observed on the northwestern limb of the solar disk on July 24th, 2016. The entire dynamic evolutionary process starting from its onset is consistent with a loop-loop interaction scenario. The X-ray profiles manifest an intriguing double-peak feature. From spectral fitting, it is found that the first peak is non-thermally dominated while the second peak is mostly multi-thermal with a hot (~10 MK) and a super-hot (~30 MK) component. This double-peak feature is unique in that the two peaks are clearly separated by 4 minutes, and the second peak reaches up to 25-50 keV; in addition, at energy bands above 3 keV the X-ray fluxes decline significantly between the two peaks. This, together with other available imaging and spectral data, manifest a two-stage energy release process. A comprehensive analysis is carried out to investigate the nature of this two-stage process. We conclude that the second stage with the hot and super-hot sources mainly involves direct heating through loop-loop reconnection at a relatively high altitude in the corona. The uniqueness of the event characteristics and complete data set make the study a nice addition to present literature on solar flares.