# Microflare heating of a solar active region observed with NuSTAR, Hinode/XRT, and SDO/AIA

, , Grefenstette, B. W., Glesener, L., Krucker, S., , Smith, D. M., Marsh, A. J., White, S. M. and Kuhar, M. (2017) Microflare heating of a solar active region observed with NuSTAR, Hinode/XRT, and SDO/AIA. Astrophysical Journal, 844(2), 132. (doi: 10.3847/1538-4357/aa7a59)

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## Abstract

NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its initial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/XRT on 2015 April 29 at ~11:29 UT. This microflare shows the heating of material to several million Kelvin, observed in soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single thermal model of 3.2–3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK, emission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at ~3 MK and falls off sharply by 5 MK; but during the microflare's impulsive phase, the emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of about $2.5\times {10}^{25}$ erg s−1. As the NuSTAR spectrum is purely thermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the accelerated electrons to be the source of heating, a power-law spectrum of $\delta \geqslant 7$ with a low-energy cutoff ${E}_{c}\lesssim 7$ keV is required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.

Item Type: Articles Published Yes Hannah, Dr Iain and Hudson, Dr Hugh and Wright, Mr Paul Wright, P. J., Hannah, I. G., Grefenstette, B. W., Glesener, L., Krucker, S., Hudson, H. S., Smith, D. M., Marsh, A. J., White, S. M., and Kuhar, M. College of Science and Engineering > School of Physics and Astronomy Astrophysical Journal IOP Publishing 0004-637X 1538-4357 31 July 2017 Copyright © 2017 The American Astronomical Society First published in Astrophysical Journal 844(2): 132 Reproduced under a Creative Commons license arXiv

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
617954The many scales of flaring energy release on the SunIain HannahThe Royal Society (ROYSOC)UF120063S&E P&A - PHYSICS & ASTRONOMY
646461EPSRC-Royal Society fellowship engagement (2013): Heating of the solar atmosphere by small flaresIain HannahEngineering and Physical Sciences Research Council (EPSRC)EP/M00371X/1S&E P&A - PHYSICS & ASTRONOMY