Quiet Sun impulsive events observed with NuSTAR during solar minimum
Sarah Paterson, Iain G. Hannah, Brian W. Grefenstette, S\"am Krucker, Erica Lastufka, Hugh S. Hudson, Lindsay Glesener, Stephen M. White, David M. Smith

TL;DR
This study used NuSTAR's high sensitivity to observe faint impulsive events in the quiet Sun during solar minimum, revealing thermal properties and energy scales that differ from active region flares, with no evidence of non-thermal emission.
Contribution
First observations of quiet Sun impulsive events with NuSTAR, providing detailed thermal parameters and upper limits on non-thermal emissions, highlighting differences from active region flares.
Findings
Seven quiet Sun flares observed with NuSTAR
Thermal energies between 2.5-8.9 x 10^{26} erg
No evidence of non-thermal emission detected
Abstract
The investigation of small-scale energy release in the Sun's atmosphere is important in understanding how the corona is heated. Previous work has been able to study small EUV and SXR brightenings outside of active regions (i.e. the quiet Sun), but with HXRs this has mostly focused on active region transients/microflares due to the sensitivity of available telescopes. In this paper we present observations of the quiet Sun with the Nuclear Spectroscopic Telescope Array (NuSTAR), an X-ray imaging spectrometer with much greater sensitivity than previous instruments, allowing the observation of faint events. During the recent solar minimum, NuSTAR captured seven quiet Sun flares/impulsive brightenings, three on 21 February 2020, and four on 12-13 September 2020. From fitting their NuSTAR HXR spectra we find temperatures of 3.1-4.0 MK and emission measures between (0.75-17.0) …
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Taxonomy
TopicsSolar and Space Plasma Dynamics · Ionosphere and magnetosphere dynamics · Earthquake Detection and Analysis
