NuSTAR observation of energy release in 11 solar microflares

Duncan, J., Glesener, L., Grefenstette, B., Vievering, J., Hannah, I. , Smith, D., Krucker, S., White, S. and Hudson, H. (2021) NuSTAR observation of energy release in 11 solar microflares. Astrophysical Journal, 908(1), 29. (doi: 10.3847/1538-4357/abca3d)

Abstract

Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several ``large flare'' properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (łt1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis.

• Item Type: Articles
• Keywords: The Sun, Solar flare spectra, Solar x-ray flares, Non-thermal radiation sources, Solar flares, Solar physics, 1693, 1982, 1816, 1119, 1496, 1476, Astrophysics - Solar and Stellar Astrophysics.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Hudson, Dr Hugh and Hannah, Dr Iain
• Authors: Duncan, J., Glesener, L., Grefenstette, B., Vievering, J., Hannah, I., Smith, D., Krucker, S., White, S., and Hudson, H.
• Subjects: Q Science > QB Astronomy
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Astrophysical Journal
• Publisher: IOP Publishing
• ISSN: 0004-637X
• ISSN (Online): 1538-4357
• Published Online: 09 February 2021
• Copyright Holders: Copyright © 2021. The American Astronomical Society
• First Published: First published in © 2021. The American Astronomical Society 908(1):29
• Publisher Policy: Reproduced in accordance with the publisher copyright policy