The efficiency of electron acceleration during the impulsive phase of a solar flare

Kontar, E. P. , Emslie, A. G., Motorina, G. G. and Dennis, B. R. (2023) The efficiency of electron acceleration during the impulsive phase of a solar flare. Astrophysical Journal Letters, 947(1), L13. (doi: 10.3847/2041-8213/acc9b7)

Abstract

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of ≲10−2 on the number density of nonthermal (≥20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

• Item Type: Articles
• Additional Information: This work was supported by STFC consolidated grant ST/T000422/1. A.G.E. was supported by NASA Kentucky under NASA award number 80NSSC21M0362. GGM was supported by grant 21-16508J of the Grant Agency of the Czech Republic, the project RVO:67985815, the project LM2018106 of the Ministry of Education, Youth and Sports of the Czech Republic, and the State Assignment 0040-2019-0025.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Motorina, Ms Galina and Kontar, Professor Eduard
• Authors: Kontar, E. P., Emslie, A. G., Motorina, G. G., and Dennis, B. R.
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Astrophysical Journal Letters
• Publisher: IOP Publishing
• ISSN: 2041-8205
• ISSN (Online): 2041-8213
• Published Online: 14 April 2023
• Copyright Holders: Copyright © 2023 The Authors
• First Published: First published in Astrophysical Journal Letters 947(1): L13
• Publisher Policy: Reproduced under a Creative Commons License

Project Code	Award No	Project Name	Principal Investigator	Funder's Name	Funder Ref	Lead Dept
306515		PHAS A&A Group STFC Consolidated	Lyndsay Fletcher	Science and Technology Facilities Council (STFC)	ST/T000422/1	P&S - Physics & Astronomy