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NuSTAR observations of a repeatedly microflaring active region

Cooper, Kristopher and Hannah, Iain G. and Grefenstette, Brian W. and Glesener, Lindsay and Krucker, Säm and Hudson, Hugh S. and White, Stephen M. and Smith, David M. and Duncan, Jessie (2021) NuSTAR observations of a repeatedly microflaring active region. Monthly Notices of the Royal Astronomical Society, 507 (3). pp. 3936-3951. ISSN 0035-8711. PMCID PMC8753558. doi:10.1093/mnras/stab2283.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Cooper, Kristopher0000-0001-8589-3378
Hannah, Iain G.0000-0003-1193-8603
Grefenstette, Brian W.0000-0002-1984-2932
Glesener, Lindsay0000-0001-7092-2703
Krucker, Säm0000-0002-2002-9180
Hudson, Hugh S.0000-0001-5685-1283
Duncan, Jessie0000-0002-6872-4406
Additional Information:© 2021 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Accepted 2021 August 3. Received 2021 July 29; in original form 2021 May 5. Published: 19 August 2021. 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). Data Availability: All data used are publicly available. SDO data can be obtained from the Joint Science Operations Center (JSOC)4 using SUNPY’SFido5 object while the NuSTAR data are available from the NuSTAR Master Catalog6 with the OBSIDs 80414201001, 80414202001, 80414203001, 80415201001, 80415202001, and 80415203001.
Group:Space Radiation Laboratory, NuSTAR
Funding AgencyGrant Number
Royal SocietyRGF\EA\180010
Royal SocietyURF\R\180010
Subject Keywords:Sun: activity – Sun: corona – Sun: flares – Sun: magnetic fields – Sun: X-rays, gamma-rays
Issue or Number:3
PubMed Central ID:PMC8753558
Record Number:CaltechAUTHORS:20211116-184723183
Persistent URL:
Official Citation:Kristopher Cooper, Iain G Hannah, Brian W Grefenstette, Lindsay Glesener, Säm Krucker, Hugh S Hudson, Stephen M White, David M Smith, Jessie Duncan, NuSTAR observations of a repeatedly microflaring active region, Monthly Notices of the Royal Astronomical Society, Volume 507, Issue 3, November 2021, Pages 3936–3951,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:111901
Deposited By: Tony Diaz
Deposited On:16 Nov 2021 20:23
Last Modified:18 Jan 2022 18:20

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