Solar Flares as Electron Accelerators: Toward a Resolution of the Acceleration Efficiency Issue

A major open issue concerning the active Sun is the effectiveness with which magnetic reconnection accelerates electrons in flares. A paper published by 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 Astrophysical Journal Letters used hard X-ray measurements 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 a set of flares. We use the spatial variation of the electron spectrum to deduce the density of the medium where electrons propagate and, from this and the total intensity of the flare, the ratio of accelerated to ambient electron densities. Results show that this ratio never exceeds 1% or so in all the events analyzed.