Hot X-ray onsets of solar flares

Hudson, H. S. , Simões, P. J.A. , Fletcher, L. , Hayes, L. A. and Hannah, I. G. (2021) Hot X-ray onsets of solar flares. Monthly Notices of the Royal Astronomical Society, 501(1), pp. 1273-1281. (doi: 10.1093/mnras/staa3664)

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The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray 'onset' interval of enhanced isothermal plasma temperatures in the range of 10-15 MK over a period of time prior to the flare's impulsive phase. This 'hot onset' interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the 'hot onset' phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

Item Type:Articles
Additional Information:(MackPesquisa), CNPq (contract 307612/2019-8), and FAPESP, the Sao Paulo Research Foundation (contract 2013/24155-3). IGH is supported by a Royal Society University Fellowship. LF acknowledges support from UK Research and Innovation’s Science and Technology Facilities Council under grant award numbers ST/P000533/1 and ST/T000422/1. LAH is supported by an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by USRA (the Universities Space Research Association) through a contract with NASA.
Keywords:Sun: corona, Sun: flares, Sun: X-rays, Astrophysics - Solar and Stellar Astrophysics
Glasgow Author(s) Enlighten ID:Simoes, Dr Paulo and Fletcher, Professor Lyndsay and Hannah, Dr Iain and Hudson, Dr Hugh
Authors: Hudson, H. S., Simões, P. J.A., Fletcher, L., Hayes, L. A., and Hannah, I. G.
Subjects:Q Science > QB Astronomy
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Monthly Notices of the Royal Astronomical Society
Publisher:Oxford University Press
ISSN (Online):1365-2966
Published Online:25 November 2020
Copyright Holders:Copyright © 2020 The Authors
First Published:First published in Monthly Notices of the Royal Astronomical Society 501(1):1273–1281
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
173869Consolidated Grant in Solar PhysicsLyndsay FletcherScience and Technology Facilities Council (STFC)ST/P000533/1P&S - Physics & Astronomy
306515PHAS A&A Group STFC ConsolidatedLyndsay FletcherScience and Technology Facilities Council (STFC)ST/T000422/1P&S - Physics & Astronomy