2018/03/22 by Matej Kuhar, Säm Krucker, Lindsay Glesener +8
Computer Science · Physics and Astronomy · #Astronomy #Astrophysics #Corona (planetary geology) #Extreme ultraviolet #Extreme ultraviolet lithography #Flux (metallurgy) #Isothermal process #Optics #Physics #QUIET #Solar Radiation and Photovoltaics #Solar and Space Plasma Dynamics #Solar flare #Spectral line #Spectroscopy #Stellar, planetary, and galactic studies #Thermal #Ultraviolet #astro-ph.HE #astro-ph.SR
paper · pdf · doi:10.3847/2041-8213/aab889
Accepted for publication in ApJL
arxiv created 2018/03/22 · openalex publication_date 2018/03/30 · arxiv updated 2018/04/11 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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) × 10 44 cm −3 describe their spectra well, resulting in thermal energies in the range (2–6) × 10 26 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.