Spectroscopic evidence of cool plasma in quiet Sun HRIEUV small scale brightenings

TL;DR

This study investigates small-scale EUV brightenings in the quiet Sun, revealing they mostly reach chromospheric and transition region temperatures, thus likely not significantly contributing to coronal heating.

Contribution

The paper provides plasma diagnostics of 11 quiet Sun EUV brightenings, demonstrating they rarely exceed 1 MK and have densities around 10^{10} cm^{-3}, clarifying their thermal role.

Findings

Events are dominated by chromospheric and TR plasma emission.

Brightenings rarely exceed 1 MK in temperature.

Estimated electron density is approximately 1.8 x 10^{10} cm^{-3}.

Abstract

Context: A large number of the small and the 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 (HRIEUV) on board Solar Orbiter. It is still uncertain whether these events reach coronal temperatures, and thus if they directly participate to coronal heating. Aims: In this work, we evaluate the maximum temperature of 11 EUV brightenings in the QS, through plasma diagnostics involving UV/EUV spectroscopy and imaging. Methods: We use three QS observations coordinated between HRIEUV, the Spectral Imaging of the Coronal Environment (SPICE/Solar Orbiter), the EUV Imaging Spectrometer (EIS/Hinode), and the Atmospheric Imaging Assembly (AIA/SDO). We detected events in HRIEUV, ranging from 0.8 to 6.2 Mm 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. Results: These events are dominated by emission of plasma at chromospheric and TR temperatures, and they barely reach temperatures above 1 MK. As such, we concluded that their contribution to coronal heating is not dominant. The estimated density of one of the event is n$_e$ = (1.8 $\pm$ 1.3) $\times$ $10^{10}$ cm$^{-3}$.