The Above-the-looptop Source of the 2017 September 10 Solar Flare: Energetic Electron Distribution over a Broad Energy Range

Abstract:

Nonthermal sources located above bright flare arcades, referred to as the "above-the-looptop" sources, have been suggested as a candidate for the primary electron energization site in major solar flares. The spectacular X8.2-class limb flare on 2017 September 10 features such an above-the-looptop source, which was observed during the early impulsive phase of the flare in both microwaves and hard X-rays. By combining spatially resolved EUV, X-ray, and microwave observations from SDO/AIA, Hinode/XRT, RHESSI, and the Expanded Owens Valley Solar Array (EOVSA), we derive the energetic electron distribution of this source over a broad energy range from <1 keV up to ~1 MeV. The electron distribution consists of a thermal "core" from 20­­–30 MK plasma that dominates the energy range below ~30 keV. Meanwhile, a nonthermal "tail" exists at above ~30 keV and extends to at least hundreds of keV, possibly with a spectral break. We discuss implications of the results for plasma heating and electron acceleration during the early development of this flare.