Solar Flare Hosts MeV-peaked Electrons in a Coronal Source

Abstract: Solar flares promptly release large amounts of free magnetic energy in the solar corona to produce substantial populations of high-energy charged particles, both ions and electrons. These particles are detected when they radiate microwaves in solar magnetic fields and X- and γ-rays when they encounter matter. Analysis of γ-rays in solar flares has revealed a distinct continuum component dominating at MeV energies, which differs from the well-studied X-ray continuum produced by flare-accelerated electrons with steeply falling energy spectra. The origin and precise spatial location and extent of this mysterious MeV component have been unknown up to now. If it is produced by bremsstrahlung, such a γ-ray component requires an unusual population of electrons peaked at a few MeV. Here we report a joint study of this MeV-peaked electron population in the 2017-Sep-10 solar flare with Fermi MeV γ-ray data and EOVSA spatially resolved microwave imaging spectroscopy data. We demonstrate that the microwave spectrum from the peaked MeV distribution has a distinctly different shape from that produced by the well-known population of electrons with falling energy spectrum. We inspected microwave maps of the flare and identified an evolving area where the measured microwave spectra matched the theoretically expected one for the MeV-peaked population, thus pinpointing the site where this MeV component resides in the flare. The locations are in a coronal volume adjacent to the region where prominent release of magnetic energy and bulk electron acceleration were detected, which implies that transport effects play a key role in forming this population.