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 3.2−4.1 MK and emission measures of (0.6−15)×1044 cm−3 describe their spectra well, resulting in thermal energies in the range (2−6)×1026 ergs. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, since the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated GOES classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them 8 orders of magnitude fainter in soft X-ray flux than the largest solar flares.
@article{arxiv.1803.08365,
title = {NuSTAR Detection of X-Ray Heating Events in the Quiet Sun},
author = {Matej Kuhar and Säm Krucker and Lindsay Glesener and Iain G. Hannah and Brian W. Grefenstette and David M. Smith and Hugh S. Hudson and Stephen M. White},
journal= {arXiv preprint arXiv:1803.08365},
year = {2018}
}