Quiet Sun impulsive events observed with NuSTAR during solar minimum

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 extreme ultraviolet and soft X-ray brightenings outside of active regions (i.e. the quiet Sun), but with hard X-rays (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 2020 February 21, and four on 2020 September 12–13. From fitting their NuSTAR HXR spectra, we find temperatures of 3.1–4.0 MK and emission measures between (0.75–17.0) $\times \,\, 10^{43}$ cm$^{-3}$, which gives thermal energies between (2.5–8.9) $\times \,\, 10^{26}$ erg. Only one event, a mini-filament eruption, showed evidence of slightly higher temperature emissions, confirmed through differential emission measure analysis. None of the events showed evidence of non-thermal emission in their NuSTAR spectra, and we placed upper limits to the accelerated electron population. The thermal parameters for these quiet Sun events seem to scale differently to previously studied active region flares, suggesting a different energy release process might be dominating. However, this conclusion is affected by the different sensitivity and biases introduced by the various instruments and analysis approaches used.