Nustar detection of x-ray heating events in the quiet sun

Matej Kuhar, Säm Krucker, Lindsay Glesener, Iain G. Hannah, Brian W. Grefenstette, David M. Smith, Hugh S. Hudson, Stephen M. White

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29 Scopus citations


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.

Original languageEnglish (US)
Article numberL32
JournalAstrophysical Journal Letters
Issue number2
StatePublished - Apr 1 2018

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  • Gamma rays
  • Sun: Flaressun: Particle emission
  • Sun: X-rays


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