NuSTAR Detection of X-Ray Heating Events in the Quiet Sun

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 $3.2-4.1$ MK and emission measures of $(0.6-15)\times10^{44} \textrm{ cm}^{-3}$ describe their spectra well, resulting in thermal energies in the range $(2-6)\times10^{26}\textrm{ ergs}$. 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 $1/1000 \textrm{ and } 1/100$ A class level, making them 8 orders of magnitude fainter in soft X-ray flux than the largest solar flares.