NuSTAR observations of a repeatedly microflaring active region

Kristopher Cooper, Iain G. Hannah, Brian W. Grefenstette, Lindsay Glesener, Säm Krucker, Hugh S. Hudson, Stephen M. White, David M. Smith, Jessie Duncan

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14 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)3936-3951
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Nov 1 2021

Bibliographical note

Funding Information:
This paper made use of data from the NuSTAR mission, a project led by the California Institute of Technology,managed by the Jet Propulsion Laboratory, funded by the National Aeronautics and Space Administration. These observations were supported through the NuSTAR Guest Observer program (NASA grant 80NSSC18K1744). This research used version 2.0.6 (Mumford et al. 2021) of the SUNPY open source software package (SunPy Community et al. 2020), version 0.4.0 (Barnes et al. 2020a) of the AIAPY open source software package (Barnes et al. 2020b), and made use of ASTROPY,3 a community-developed core PYTHON package for Astronomy (The Astropy Collaboration 2018). Other PYTHON packages that were extensively used were MATPLOTLIB (Hunter 2007), NUMPY (Harris et al. 2020), and SCIPY (Virtanen et al. 2020). This research also made use of HEASOFT (a unified release of FTOOLS and XANADU software packages) and NuSTAR Data Analysis Software (NUSTARDAS). This paper also made use of the SolarSoft IDL distribution (SSW) from the IDL Astronomy Library. We would like to thank the anonymous reviewer for their helpful feedback. KC is supported by a Royal Society Research Fellows Enhancement Award (RGF\EA\180010) and IGH is supported by a Royal Society University Fellowship (URF\R\180010).

Publisher Copyright:
© 2021 The Author(s) Published by Oxford University Press on behalf of The Royal Astronomical Society.


  • Sun: X-rays, gamma-rays
  • Sun: activity
  • Sun: corona
  • Sun: flares
  • Sun: magnetic fields


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