NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
Matej Kuhar, S\"am Krucker, Lindsay Glesener, Iain G. Hannah, Brian W., Grefenstette, David M. Smith, Hugh S. Hudson, Stephen M. White

TL;DR
This study presents the first imaging spectroscopy X-ray observations of quiet Sun flares, revealing low-temperature, thermal events with no detectable nonthermal components, contributing to understanding nanoflares and coronal heating.
Contribution
First imaging spectroscopy X-ray observations of quiet Sun flares, providing detailed thermal characterization and constraints on nonthermal energy contributions.
Findings
Flares showed low temperatures (3.2-4.1 MK) and thermal energies (2-6 x 10^{26} ergs)
No signs of nonthermal or higher temperature components in spectra
Flares are 8 orders of magnitude fainter than large solar flares
Abstract
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occuring across the whole solar disk. In this paper, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the NuSTAR solar campaigns on 2016 July 26 and 2017 March 21, concurrent with SDO/AIA observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet (EUV) emissions. Isothermal fits with rather low temperatures in the range MK and emission measures of describe their spectra well, resulting in thermal energies in the range . NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, since the estimated upper limits of (hidden) nonthermal…
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