NuSTAR observations of a repeatedly microflaring active region. Issue 3 (19th August 2021)

• Record Type: Journal Article
• Authors: Cooper, Kristopher; Hannah, Iain G; Grefenstette, Brian W; Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S; White, Stephen M; Smith, David M; Duncan, Jessie
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 507:Issue 3(2021)
• Page Start: 3936
• Page End: 3951
• Publication Date: 2021-08-19
• Subjects: Sun: activity -- Sun: corona -- Sun: flares -- Sun: magnetic fields -- Sun: X-rays, gamma-rays; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab2283
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We investigate the spatial, temporal, and spectral properties of 10 microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager. We find GOES sub-A class equivalent microflare energies of 10 26 –10 28 erg reaching temperatures up to 10 MK with consistent quiescent or hot active region (AR) core plasma temperatures of 3–4 MK. One microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has non-thermal hard X-ray emission during its impulsive phase (of non-thermal power ∼7 × 10 24 erg s −1 ) making it one of the faintest X-ray microflares to have direct evidence for accelerated electrons. In 4 of the 10 microflares, we find that the X-ray time profile matches fainter and more transient sources in the extreme-ultraviolet, highlighting the need for observations sensitive to only the hottest material that reaches temperatures higher than those of the AR core (>5 MK). Evidence for corresponding photospheric magnetic flux cancellation/emergence present at the footpoints of eight microflares is also observed.