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 1026-1028 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 × 1024 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.
Highlights
Active regions (ARs) are observed to sometimes produce repeated flares across decades of energies
In this paper we present the largest study of microflares occurring in a single active region observed with Nuclear Spectroscopic Telescope ARray (NuSTAR)
We significantly increase the number of investigated NuSTAR microflares in current literature helping to provide a more statistical view of flares and their nature at this scale
Summary
Active regions (ARs) are observed to sometimes produce repeated flares across decades of energies. To identify features of interest for analysis from AR12721 time profiles were produced over the six dwell times from four SDO/AIA channels, the Fe proxy, and NuSTAR grade 0 (single pixel) FPMA+B X-ray counts >2.5 keV (Figure 1). This reveals the presence of 10 microflares over the first 5 orbits (labelled 1–10) in X-rays with only weak correlations in the the native SDO/AIA channels. Microflare 4 in Figure 1 (panel c), was previously found to be the weakest X-ray microflare in literature (Cooper et al 2020)
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