Quiet Sun impulsive events observed with NuSTAR during solar minimum

TL;DR

The study uses NuSTAR–a highly sensitive hard X-ray imaging spectrometer–to characterize seven impulsive quiet-Sun events during solar minimum, aiming to understand coronal heating without active regions. Spectral fits yield temperatures between $3.1$ and $4.0$ MK and emission measures of $(0.75-17)\times10^{43}$ cm$^{-3}$, with thermal energies of $(2.5-8.9)\times10^{25}$ erg; no robust non-thermal emission is detected, and only upper limits on accelerated electrons remain. A subset shows multi-thermal signatures via Differential Emission Measure analysis, notably Event 2 which evidences plasma above $4$ MK. The quiet-Sun events appear cooler and less energetic than active-region flares, suggesting a potentially different energy-release mechanism, though instrument biases and analysis choices temper definitive conclusions. Overall, NuSTAR reveals the faint end of solar HXRs in the quiet Sun and highlights the need for next-generation solar HXRs to better constrain heating versus particle acceleration in these events.

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) $\times 10^{43}$ 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 temperatures emission, 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.

Figures (8)

• Figure 1: An overview of the three impulsive quiet Sun events (labelled Events 1--3) captured with NuSTAR on 21 February 2020. Top row: AIA 211Å images of the flaring regions, with NuSTAR FPMA + FPMB contours shown in green. The AIA light curves were found over the grey dashed boxes. Bottom row: NuSTAR and AIA light curves showing the X-ray and EUV evolution of the events. Shaded regions represent the time intervals used for NuSTAR spectral fitting.
• Figure 2: AIA 211Å images showing the EUV evolution of the small mini-filament eruption captured by NuSTAR on 21 February 2020 (Event 2). The blue contours in the middle panel shows the 25% level of the AIA Fe XVIII proxy channel.
• Figure 3: An overview of the four impulsive quiet Sun events (labelled Events 4--7) captured with NuSTAR on 12--13 September 2020. First and third rows: AIA 211Å images of the flaring regions, with NuSTAR FPMA + FPMB contours shown in green. The AIA light curves were found over the grey dashed boxes. Second and fourth rows: NuSTAR, XRT, and AIA light curves showing the X-ray and EUV evolution of the events. Shaded regions represent the time intervals used for NuSTAR spectral fitting. Note that the main spike in the Event 7 time profile about 00:10 UT (bottom right panel) is due to a source outside the FOV.
• Figure 4: The NuSTAR FPMA+B spectrum for the mini-filament eruption. Left: the spectrum is fitted with an isothermal model (red). Right: the spectrum is fitted with a double thermal model (purple; the two separate thermal components are shown in red and blue). Temperatures and emission measures (as well as the multiplicative factor between FPMA and FPMB) as noted on the plots.
• Figure 5: DEMs of impulsive events from February 2020 (top panel) and September 2020 (bottom panel), for the time ranges given in Table \ref{['tab:fitresults']} - when more than one time is given the first is used - and shown by the blue shaded regions in Fig. \ref{['fig:feboverview']} and Fig. \ref{['fig:sepoverview']}. Note that Event 2 from February 2020 is also shown in the September 2020 panel, greyed out, for comparison.