Hard X-ray footpoint sizes and positions as diagnostics of flare accelerated energetic electrons in the low solar atmosphere

TL;DR

This study uses RHESSI observations to analyze the sizes and positions of solar flare X-ray footpoints, revealing energy-dependent height trends and sizes that challenge existing models of electron transport in the solar atmosphere.

Contribution

It provides detailed measurements of HXR footpoint positions and sizes across energies, highlighting discrepancies with the collisional thick-target model and exploring potential explanations.

Findings

Half of the events show decreasing source height with energy.

HXR sources are located 600-1200 km above the photosphere.

Vertical sizes are up to four times larger than model predictions.

Abstract

The hard X-ray (HXR) emission in solar flares comes almost exclusively from a very small part of the flaring region, the footpoints of magnetic loops. Using RHESSI observations of solar flare footpoints, we determine the radial positions and sizes of footpoints as a function of energy in six near-limb events to investigate the transport of flare accelerated electrons and the properties of the chromosphere. HXR visibility forward fitting allows to find the positions/heights and the sizes of HXR footpoints along and perpendicular to the magnetic field of the flaring loop at different energies in the HXR range. We show that in half of the analyzed events, a clear trend of decreasing height of the sources with energy is found. Assuming collisional thick-target transport, HXR sources are located between 600 and 1200 km above the photosphere for photon energies between 120 and 25 keV respectively. In the other events, the position as a function of energy is constant within the uncertainties. The vertical sizes (along the path of electron propagation) range from 1.3 to 8 arcseconds which is up to a factor 4 larger than predicted by the thick-target model even in events where the positions/heights of HXR sources are consistent with the collisional thick-target model. Magnetic mirroring, collisional pitch angle scattering and X-ray albedo are discussed as potential explanations of the findings.