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arXiv · 1706.06108

Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA

Paul J. Wright·Iain G. Hannah·Brian W. Grefenstette·Lindsay Glesener·Säm Krucker·Hugh S. Hudson·David M. Smith·Andrew J. Marsh·Stephen M. White·Matej Kuhar
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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 heating of material to several million Kelvin, observed in Soft X-rays (SXRs) with Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA. For three of the four NuSTAR observations of this region (pre-, decay, and post 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 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 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 the heating requires a power-law spectrum of $δ\ge 7$ with a low energy cut-off $E_{c} \lesssim 7$ keV. In summary, this first NuSTAR microflare strongly resembles much more powerful flares.

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Paul J. Wright, Iain G. Hannah, Brian W. Grefenstette, Lindsay Glesener, Säm Krucker, Hugh S. Hudson, David M. Smith, Andrew J. Marsh, Stephen M. White, Matej Kuhar. 2017-07-12. Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA. https://doi.org/10.3847/1538-4357%2Faa7a59

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