title: Searching for impulsive hard x-ray emission from the quiet sun
Creators: name: Paterson, Sarah
Publisher: name: University of Glasgow
Year: 2024

description: The temperature of the solar corona is orders of magnitude hotter than that of the surface of the Sun. This is referred to as the coronal heating problem, and one of the leading theories to solve this is that a multitude of very small-scale energy release events called nanoflares, too small to resolve individually, could together sustain the high temperature of the corona. Nanoflare heating models predict a high temperature (&gt; 5 MK) component and, if operating through similar mechanisms to large flares, they would also produce non-thermal emission due to accelerated electrons. Searching for these signatures in small-scale phenomena requires the investigation of their hard X-ray (HXR) emission.

In this thesis, we analyse HXR observations of small-scale sources in the quiet Sun. In Chapter 1, we introduce the coronal heating problem, as well as the types of small-scale phenomena that are typically found in the quiet Sun. We also give a brief overview of the mechanisms which produce solar HXR emission, and the models used to describe it. In Chapter 2, we introduce the instruments whose data is used in this thesis. This includes the Nuclear Spectroscopic Telescope Array (NuSTAR), a sensitive HXR focusing telescope which, though designed as an astrophysics mission, has observed the Sun at ∼ 2–79 keV.

During the recent solar minimum (2018–2020), NuSTAR observed the quiet Sun a number of times, and these campaigns are summarised in Chapter 3. The absence of bright sources on the disk at this point provided the unique opportunity to observe their faint HXR emission for the first time with a sensitive HXR imaging spectrometer, allowing a search for high temperature and non-thermal components. In Chapter 3, we also introduce the data analysis methods used throughout this thesis, including NuSTAR HXR spectroscopy, differential emission measure (DEM) analysis, and calculating NuSTAR non-thermal upper limits.

In Chapter 4, we analyse a variety of quiet Sun HXR sources which were captured in the NuSTAR full-disk mosaics from 28 September 2018. Among these are several X-ray bright points, an emerging flux region, and a jet. By fitting their NuSTAR spectra, we find that they all have temperatures lying in the narrow range between 2–3.2 MK. DEM analysis confirms the presence of no significant emission at temperatures &gt; 4 MK. We find no evidence of high temperature or non-thermal components, though we obtain upper limits on the non-thermal emission consistent with a null detection.

The full-disk mosaic observations are limited by their short ∼ 100 s pointings, which result in noisy spectra and are not ideal for studying time evolution of HXR sources. In Chapter 5, we instead focus on two X-ray bright points observed over several hours during NuSTAR quiet Sun dwell observations, one on 21 February 2020 and another on 12–13 September 2020. We study their time evolution, finding that both show significant HXR variability over the observations. We find that the February 2020 bright point produces “flares”, observed as spikes in the HXR emission. During flaring times, this bright point reaches temperatures &gt; 4 MK, hotter than previous temperatures found for X-ray bright points. Conversely, fitting the NuSTAR spectra of the September bright point during times of Xrays spikes reveals no hotter component during these times, only an increase in emission measure of material at ∼ 2.6 MK. DEM analysis confirms that there are no significant temperature increases.

In this chapter, we also fit the two bright points’ spectra integrated over several hours. We find that these spectra are dominated by a thermal component with a temperature of 2.6–3.2 MK at energies &lt; 4 keV. At energies higher than this, the NuSTAR instrumental background dominates. No significant hot or non-thermal component is detected.

In addition to capturing the time evolution of persisting sources, NuSTAR’s dwell observation mode also increases the chance of detecting transients. In Chapter 6, we identify seven small scale impulsive quiet Sun events captured in the 21 February and 12–13 September 2020 observing campaigns, one of which is a mini-filament eruption. From fitting their NuSTAR HXR spectra, we find that their temperatures range from 3.3–4.0 MK. In general, no hot or non-thermal components are detected in these short-duration events. However, through NuSTAR spectral analysis for the mini-filament eruption, we find evidence of emission up to temperatures of ∼ 5 MK. DEM analysis confirms that this event had more material heated to temperatures &gt; 2 MK than the other events. Again, as no non-thermal component is directly observed, we obtain upper limits on the non-thermal emission consistent with a null detection. We find that the non-thermal distribution would have to be very steep between 3–4 keV to produce the observed heating.