Megaelectronvolt-peaked electrons in a coronal source of a solar flare

Analysis of γ-rays in solar flares has suggested a distinct continuum component dominating at megaelectronvolt energies, which differs from the well-studied X-ray continuum produced by flare-accelerated electrons, with spectra steeply falling with energy. The origin, precise spatial location and extent of this mysterious megaelectronvolt 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 million electron volts. Here we report a joint study of this megaelectronvolt-peaked electron population in the 2017 September 10 solar flare with Fermi megaelectronvolt γ-ray data and spatially resolved microwave imaging spectroscopy data obtained by the Expanded Owens Valley Solar Array. We demonstrate that the microwave spectrum from the megaelectronvolt-peaked distribution has a distinctly different shape from that produced by the electrons with a falling energy spectrum. We inspected microwave maps of the flare and identified an evolving area where the measured microwave spectra matched the theoretically expected ones for the megaelectronvolt-peaked population, thus pinpointing the site where this megaelectronvolt component resides. The locations are in a coronal volume adjacent to the region where prominent release of magnetic energy and bulk electron acceleration were detected. The results imply that transport effects play a key role in forming this population of high-energy particles, which is crucial for building a complete picture of the multifaceted solar flare phenomena.