High time resolution observations of solar H-alpha flares - II. Search for signatures of electron beam heating

TL;DR

This study investigates the timing of H-alpha and hard X-ray emissions during solar flares to identify signatures of electron beam heating, revealing that most H-alpha emissions lag HXR by 1-2 seconds, indicating rapid energy transfer mechanisms.

Contribution

It provides a detailed analysis of time delays between H-alpha and HXR emissions in solar flares, extending previous work with a larger dataset and higher temporal resolution.

Findings

Most H-alpha emissions lag HXR by 1-2 seconds

Some flares show longer delays of 10-18 seconds

Short delays support electron beam heating hypothesis

Abstract

Aims. The H-alpha emission of solar flare kernels and associated hard X-ray (HXR) emission often show similar time variations but their light curves are shifted in time by energy transfer mechanisms. We searched for fast radiative response of the chromosphere in the H-alpha line as a signature of electron beam heating. Methods. We investigate the time differences with sub-second resolution between the H-alpha line emission observed with a Multi-channel Subtractive Double Pass (MSDP) spectrograph on the Large Coronagraph and Horizontal Telescope at Bialkow Observatory, Poland, and HXR emission recorded by the RHESSI spacecraft during several flares, greatly extending our earlier analysis (Paper I) to flares between 2003 and 2005. Results. For 16 H-alpha flaring kernels, observed in 12 solar flares, we made 72 measurements of time delays between local maxima of the RHESSI X-ray and H-alpha emissions. For most kernels, there is an excellent correlation between time variations in the H-alpha line emission (at line centre and in the line wings) and HXR (20-50 keV) flux, with the H-alpha emission following features in the HXR light curves generally by a short time lapse delta t = 1-2 s, sometimes significantly longer (10-18 s). We also found a strong spatial correlation. Conclusions. Owing to our larger number of time measurements than in previous studies, the distribution of delta t values shows a much clearer pattern, with many examples of short (1-2 s) delays of the H-alpha emission, but with some flares showing longer (10-18 s) delays. The former are consistent with energy transfer along the flaring loop legs by non-thermal electron beams, the latter to the passage of conduction fronts.