Spectroscopic evidence of cool plasma in quiet Sun small-scale brightenings detected by HRIEUV on board Solar Orbiter

Abstract

A large number of small and short-lived EUV brightenings have been detected in the quiet Sun (QS) over the past three years by the High Resolution Imager of the Extreme Ultraviolet Imager ( on board Solar Orbiter. It remains uncertain whether these events reach coronal temperatures and, thus, whether they directly contribute to coronal heating. In this work, we evaluate the maximum temperature of 11 EUV brightenings in the QS through plasma diagnostics with UV/EUV spectroscopy and imaging. We used three QS observations coordinated alongside those of Spectral Imaging of the Coronal Environment (SPICE/Solar Orbiter), EUV Imaging Spectrometer (EIS/Hinode), and Atmospheric Imaging Assembly (AIA/SDO). We detected events in ranging from to meter in length. We then identified nine of them in SPICE and AIA, as well as three in EIS. We investigated their temporal evolution using their light curves and applied temperature diagnostics, such as the LOCI emission measure (EM) and the differential EM (DEM). We also estimated the electron density of one event identified in EIS. These events are dominated by emission of plasma at chromospheric and TR temperatures and they barely reach temperatures above kelvin . As such, we concluded that their contribution to coronal heating is not dominant. The estimated density of one of the event is $n_ e separate-uncertainty = true 1.8(13)e10