Hot X-ray onsets of solar flares

Author(s)

Hudson, Hugh S. ^{[1, 2]} ; Simoes, Paulo J. A. ^{[2, 3]} ; Fletcher, Lyndsay ^{[2, 4]} ; Hayes, Laura A. ^[5] ; Hannah, Iain G. ^[2]

Affiliation

^[1] Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 - USA

^[2] Univ Glasgow, SUPA Sch Phys & Astron, Glasgow G12 8QQ, Lanark - Scotland

^[3] Univ Presbiteriana Mackenzie, Ctr Radio Astron & Astrofis Mackenzie, Escola Engn, BR-01302 Sao Paulo - Brazil

^[4] Univ Oslo, Rosseland Ctr Solar Phys, POB 1029 Blindern, NO-0315 Oslo - Norway

^[5] NASA, Solar Phys Lab, Heliophys Sci Div, Goddard Space Flight Ctr, Code 671, Greenbelt, MD 20771 - USA

• Document type: Journal article
• Source: Monthly Notices of the Royal Astronomical Society; v. 501, n. 1, p. 1273-1281, FEB 2021.
• Web of Science Citations: 0

Abstract

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. (AU)

• FAPESP's process: 13/24155-3 - Solar flare diagnostic in an unprecedented frequency range from microwaves to THz frequencies: challenges for interpretation (flat)
• Grantee: Carlos Guillermo Giménez de Castro
• Support Opportunities: Research Projects - Thematic Grants