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

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 $(0.6-15)\times 10^{44}{\text{ cm}}^{-3}$ describe their spectra well, resulting in thermal energies in the range $(2-6)\times 10^{26}\text{ ergs}$. 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 $1/1000\text{ and }1/100$ A class level, making them 8 orders of magnitude fainter in soft X-ray flux than the largest solar flares.

Accepted for publication in ApJL

References in corpus (5)

• RHESSI Microflare Statistics II. X-ray Imaging, Spectroscopy & Energy Distributions
• Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
• The First Focused Hard X-ray Images of the Sun with NuSTAR
• NuSTAR hard X-ray observation of a sub-A class solar flare
• First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events

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