The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare
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 t...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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IOP Publishing
2023-01-01
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| Series: | The Astrophysical Journal Letters |
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| Online Access: | https://doi.org/10.3847/2041-8213/acc9b7 |
| _version_ | 1797389758991695872 |
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| author | Eduard P. Kontar A. Gordon Emslie Galina G. Motorina Brian R. Dennis |
| author_facet | Eduard P. Kontar A. Gordon Emslie Galina G. Motorina Brian R. Dennis |
| author_sort | Eduard P. Kontar |
| collection | DOAJ |
| description | 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. |
| first_indexed | 2024-03-08T23:01:39Z |
| format | Article |
| id | doaj.art-8ff36933591545ca8431812389206222 |
| institution | Directory Open Access Journal |
| issn | 2041-8205 |
| language | English |
| last_indexed | 2024-03-08T23:01:39Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal Letters |
| spelling | doaj.art-8ff36933591545ca84318123892062222023-12-15T18:33:34ZengIOP PublishingThe Astrophysical Journal Letters2041-82052023-01-019471L1310.3847/2041-8213/acc9b7The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar FlareEduard P. Kontar0https://orcid.org/0000-0002-8078-0902A. Gordon Emslie1https://orcid.org/0000-0001-8720-0723Galina G. Motorina2https://orcid.org/0000-0001-7856-084XBrian R. Dennis3https://orcid.org/0000-0001-8585-2349School of Physics & Astronomy, University of Glasgow , Scotland, UKDepartment of Physics and Astronomy, Western Kentucky University , Bowling Green, KY 42101, USACentral Astronomical Observatory at Pulkovo of Russian Academy of Sciences , St. Petersburg, 196140, Russia; Astronomical Institute of the Czech Academy of Sciences , 251 65 Ondřejov, Czech RepublicSolar Physics Laboratory, Code 671, Heliophysics Science Division, NASA Goddard Space Flight Center , Greenbelt, MD 20771, USASolar 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.https://doi.org/10.3847/2041-8213/acc9b7Solar flaresSolar physicsSolar activitySolar x-ray flaresSolar radio flares |
| spellingShingle | Eduard P. Kontar A. Gordon Emslie Galina G. Motorina Brian R. Dennis The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare The Astrophysical Journal Letters Solar flares Solar physics Solar activity Solar x-ray flares Solar radio flares |
| title | The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare |
| title_full | The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare |
| title_fullStr | The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare |
| title_full_unstemmed | The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare |
| title_short | The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare |
| title_sort | efficiency of electron acceleration during the impulsive phase of a solar flare |
| topic | Solar flares Solar physics Solar activity Solar x-ray flares Solar radio flares |
| url | https://doi.org/10.3847/2041-8213/acc9b7 |
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