The First Survey of Quiet Sun Features Observed in Hard X-Rays with NuSTAR

Creators: Paterson, Sarah; Hannah, Iain G.; Grefenstette, Brian W.; Hudson, Hugh S.; Krucker, Säm; Glesener, Lindsay; White, Stephen M.; Smith, David M.

Abstract

We present the first survey of quiet Sun features observed in hard X-rays (HXRs), using the Nuclear Spectroscopic Telescope ARray (NuSTAR), a HXR focusing optics telescope. The recent solar minimum, combined with NuSTAR's high sensitivity, has presented a unique opportunity to perform the first HXR imaging spectroscopy on a range of features in the quiet Sun. By studying the HXR emission of these features, we can detect or constrain the presence of high temperature (> 5 MK) or non-thermal sources, to help understand how they relate to larger, more energetic solar phenomena, and determine their contribution to heating the solar atmosphere. We report on several features observed in the 28 September 2018 NuSTAR full-disk quiet Sun mosaics, the first of the NuSTAR quiet Sun observing campaigns, which mostly include steady features of X-ray bright points and an emerging flux region, which later evolved into an active region, as well as a short-lived jet. We find that the features' HXR spectra are well fitted with isothermal models with temperatures ranging between 2.0 – 3.2 MK. Combining the NuSTAR data with softer X-ray emission from Hinode/XRT and EUV from SDO/AIA, we recover the differential emission measures, confirming little significant emission above 4 MK. The NuSTAR HXR spectra allow us to constrain the possible non-thermal emission that would still be consistent with a null HXR detection. We found that for only one of the features (the jet) was there a potential non-thermal upper limit capable of powering the heating observed. However, even here, the non-thermal electron distribution had to be very steep (effectively mono-energetic) with a low energy cut-off between 3 – 4 keV.

Additional Information

© The Author(s) 2023. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. This paper made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research made use of the NuSTAR Data Analysis Software (NUSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and UKSA as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway). AIA on the Solar Dynamics Observatory is part of NASA's Living with a Star program. This research has made use of SunPy, an open-source and free community-developed solar data analysis package written in Python (SunPy Community et al., 2015). This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al., 2018, 2013). The authors would also like to thank Kathy Reeves for co-ordinating the Hinode/XRT observations. SP acknowledges support from the UK's Science and Technology Facilities Council (STFC) doctoral training grant (ST/T506102/1). IGH acknowledges support from a Royal Society University Fellowship (URF/R/180010) and STFC grant (ST/T000422/1). Contributions. SP did the data processing and applied the analysis techniques described and wrote the manuscript text. IGH developed the demreg analysis techniques and helped with the spectral analysis. All authors revised the manuscript. The authors declare no competing interests.

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