Energetic Electron Distribution of the Coronal Acceleration Region: First results from Joint Microwave and Hard X-ray Imaging Spectroscopy

Abstract

Nonthermal sources located above bright flare arcades, referred to as the "above-the-loop-top" sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array (EOVSA) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), respectively. By combining the microwave and HXR imaging spectroscopy observations with multi-filter extreme ultraviolet and soft X-ray imaging data, we derive the energetic electron distribution of this source over a broad energy range from $<$10 keV up to $\sim$MeV during the early impulsive phase of the flare. The best-fit electron distribution consists of a thermal "core" from $\sim$25 MK plasma. Meanwhile, a nonthermal power-law "tail" joins the thermal core at $\sim$16 keV with a spectral index of $\sim$3.6, which breaks down at above $\sim$160 keV to $>$6.0. In addition, temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from $>$20 to $\sim$6.0 within 20 s, or less than $\sim$10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an on-going bulk acceleration of energetic electrons is taking place very early in the flare energy release.

12 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters

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Cited by in corpus (10)

• Modeling Electron Acceleration and Transport in the Early Impulsive Phase of the 2017 September 10 Solar Flare
• Numerical Modeling of Energetic Electron Acceleration, Transport, and Emission in Solar Flares: Connecting Loop-top and Footpoint Hard X-Ray Sources
• Microwave Imaging of Quasi-periodic Pulsations at Flare Current Sheet
• The efficiency of electron acceleration during the impulsive phase of a solar flare
• Multifrequency microwave imaging of weak transients from the quiet solar corona
• Implications for additional plasma heating driving the extreme-ultraviolet late phase of a solar flare with microwave imaging spectroscopy
• Observations of Thomson scattering from a loop-prominence system
• Coupling between magnetic reconnection, energy release, and particle acceleration in the X17.2 2003 October 28 solar flare
• Non-thermal Sources from Stereoscopic Hard X-ray and Earth-based Microwave Observations in a Data-Constrained Magnetohydrodynamic Simulation
• X-ray fine structure of a limb solar flare revealed by Insight-HXMT, RHESSI and Fermi