Impulsive Thermal X-ray Emission from a Low-lying Coronal Loop

TL;DR

This study analyzes a low-lying solar microflare, revealing that its impulsive X-ray emission is hot and quasi-thermal, challenging traditional models and suggesting dense loop involvement with minimal chromospheric footpoint emission.

Contribution

It provides detailed observations of a microflare with impulsive X-ray emission originating from a low-lying, dense loop, challenging conventional non-thermal flare models.

Findings

Impulsive X-ray emission is consistent with a hot, quasi-thermal origin.

Minimal evidence of chromospheric footpoint emission.

The flare occurs in a very low or chromospheric loop.

Abstract

Understanding the relationship among different emission components plays an essential role in the study of particle acceleration and energy conversion in solar flares. In flares where gradual and impulsive emission components can be readily identified the impulsive emission has been attributed to non-thermal particles. We carry out detailed analysis of H\alpha\ and X-ray observations of a GOES class B microflare loop on the solar disk. The impulsive hard X-ray emission, however, is found to be consistent with a hot, quasi-thermal origin, and there is little evidence of emission from chromospheric footpoints, which challenges conventional models of flares and reveals a class of microflares associated with dense loops. H\alpha\ observations indicate that the loop lies very low in the solar corona or even in the chromosphere and both emission and absorption materials evolve during the flare. The enhanced H\alpha\ emission may very well originate from the photosphere when the low-lying flare loop heats up the underlying chromosphere and reduces the corresponding H\alpha\ opacity. These observations may be compared with detailed modeling of flare loops with the internal kink instability, where the mode remains confined in space without apparent change in the global field shape, to uncover the underlying physical processes and to probe the structure of solar atmosphere.