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

Kuhar, Matej and Krucker, Säm and Glesener, Lindsay and Hannah, Iain G. and Grefenstette, Brian W. and Smith, David M. and Hudson, Hugh S. and White, Stephen M. (2018) NuSTAR Detection of X-Ray Heating Events in the Quiet Sun. Astrophysical Journal Letters, 856 (2). Art. No. L32. ISSN 2041-8213. doi:10.3847/2041-8213/aab889.

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The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2–4.1 MK and emission measures of (0.6–15) × 10^(44) cm^(−3) describe their spectra well, resulting in thermal energies in the range (2–6) × 10^(26) erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as 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 Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Kuhar, Matej0000-0002-7210-180X
Glesener, Lindsay0000-0001-7092-2703
Hannah, Iain G.0000-0003-1193-8603
Grefenstette, Brian W.0000-0002-1984-2932
Smith, David M.0000-0002-0542-5759
Hudson, Hugh S.0000-0001-5685-1283
White, Stephen M.0000-0002-8574-8629
Additional Information:© 2018 The American Astronomical Society. Received 2017 December 18; revised 2018 March 20; accepted 2018 March 21; published 2018 March 30. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. M.K. and S.K. acknowledge funding from the Swiss National Science Foundation (200021-140308). I.G.H. is supported by a Royal Society University Research Fellowship. L.G. was supported by an NSF Faculty Development Grant (AGS-1429512). We thank the referee for the thorough reading of the manuscript and the helpful comments that substantially improved the paper.
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Swiss National Science Foundation (SNSF)200021-140308
Subject Keywords:Sun: flares – Sun: particle emission – Sun: X-rays, gamma rays
Issue or Number:2
Record Number:CaltechAUTHORS:20180330-111231496
Persistent URL:
Official Citation:Matej Kuhar et al 2018 ApJL 856 L32
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85533
Deposited By: Tony Diaz
Deposited On:30 Mar 2018 19:22
Last Modified:15 Nov 2021 20:29

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