NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
Abstract
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2-4.1 MK and emission measures of (0.6-15) × 1044 cm-3 describe their spectra well, resulting in thermal energies in the range (2-6) × 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2018
- DOI:
- 10.3847/2041-8213/aab889
- arXiv:
- arXiv:1803.08365
- Bibcode:
- 2018ApJ...856L..32K
- Keywords:
-
- Sun: flares;
- Sun: particle emission;
- Sun: X-rays;
- gamma rays;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for publication in ApJL