Solar flares as electron accelerators: toward a resolution of the acceleration efficiency issue

TL;DR

This paper investigates the efficiency of electron acceleration during solar flares by analyzing spatially resolved X-ray observations, resolving a controversy over whether nearly all or only a small fraction of electrons are accelerated.

Contribution

It introduces a spectral inversion method combined with spatially resolved data to accurately determine the electron acceleration efficiency in solar flares.

Findings

The ratio of accelerated to ambient electrons never exceeds about 1%.

Spatially resolved observations clarify the actual acceleration efficiency.

Results challenge the view that the Sun accelerates nearly all electrons in flares.

Abstract

A major open issue concerning the active Sun is the effectiveness with which magnetic reconnection accelerates electrons in flares. A paper published by {\em{Nature}} in 2022 used microwave observations to conclude that the Sun is an almost ideal accelerator, energizing nearly all electrons within a coronal volume to nonthermal energies. Shortly thereafter, a paper published in {\em{Astrophysical Journal Letters}} used hard X-ray measurements \emph{of the same event} to reach the contradictory conclusion that less than 1\% of the available electrons were accelerated. Here we address this controversy by using spatially resolved observations of hard X-ray emission and a spectral inversion method to determine the evolution of the electron spectrum throughout the flare. So we estimated the density of the medium where electrons accelerate and, from this, the ratio of accelerated to ambient electron densities. Results show that this ratio never exceeds a percent or so in the cases analyzed.