NuSTAR detection of X-ray heating events in the quiet Sun

Kuhar, M., Krucker, S., Glesener, L., Hannah, I. G. , Grefenstette, B. W., Smith, D. M., Hudson, H. S. and White, S. M. (2018) NuSTAR detection of X-ray heating events in the quiet Sun. Astrophysical Journal Letters, 856(2), L32. (doi:10.3847/2041-8213/aab889)

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Abstract

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) × 1044 cm−3 describe their spectra well, resulting in thermal energies in the range (2–6) × 1026 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:Articles
Additional Information: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 (AGS1429512).
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Hudson, Dr Hugh and Hannah, Dr Iain
Authors: Kuhar, M., Krucker, S., Glesener, L., Hannah, I. G., Grefenstette, B. W., Smith, D. M., Hudson, H. S., and White, S. M.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal Letters
Publisher:American Astronomical Society
ISSN:2041-8205
ISSN (Online):2041-8213
Published Online:30 March 2018
Copyright Holders:Copyright © 2018 The American Astronomical Society
First Published:First published in Astrophysical Journal Letters 856(1):L32
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
617954The many scales of flaring energy release on the SunIain HannahThe Royal Society (ROYSOC)UF120063S&E P&A - PHYSICS & ASTRONOMY