NuSTAR observations of a repeatedly microflaring active region

Author:

Cooper Kristopher1ORCID,Hannah Iain G1,Grefenstette Brian W2,Glesener Lindsay3,Krucker Säm45,Hudson Hugh S15ORCID,White Stephen M6,Smith David M7,Duncan Jessie3

Affiliation:

1. School of Physics & Astronomy, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK

2. Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA

3. School of Physics & Astronomy, University of Minnesota Twin Cities, Minneapolis, MN 55455, USA

4. School of Engineering, University of Applied Sciences and Arts Northwestern Switzerland, CH-5210 Windisch, Switzerland

5. Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA

6. Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, NM 87123, USA

7. Santa Cruz Institute of Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA

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 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.

Funder

National Aeronautics and Space Administration

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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