Plasma heating in the very early phase of solar flares

TL;DR

This study investigates the early phase of a solar flare by analyzing X-ray emissions, demonstrating that non-thermal electron beams can account for initial heating and soft X-ray emission, using hydrodynamic modeling.

Contribution

It introduces a hydrodynamic model driven by non-thermal electron beams to explain early flare heating and soft X-ray emission in solar flares.

Findings

Non-thermal electron beams are sufficient to heat flare loops during rise phase.

Soft X-ray emission precedes main hard X-ray emission by about 5 minutes.

Electron beam parameters change dynamically during the flare.

Abstract

In this paper we analyze soft and hard X-ray emission of the 2002 September 20 M1.8 GOES class solar flare observed by RHESSI and GOES satellites. In this flare event, soft X-ray emission precedes the onset of the main bulk hard X-ray emission by ~5 min. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However RHESSI spectra indicate presence of the non-thermal electrons also before impulsive phase. So, we assumed that a dominant energy transport mechanism during rise phase of solar flares is electron beam-driven evaporation. We used non-thermal electron beams derived from RHESSI spectra as the heating source in a hydrodynamic model of the analyzed flare. We showed that energy delivered by non-thermal electron beams is sufficient to heat the flare loop to temperatures in which it emits soft X-ray closely following the GOES 1-8 A light-curve. We also analyze the number of non-thermal electrons, the low energy cut-off, electron spectral indices and the changes of these parameters with time.