Prevalence of Thermal Nonequilibrium over an Active Region

Abstract

Recent observations have shown that besides the characteristic multimillion degree component, the corona also contains a large amount of cool material called coronal rain, whose clumps are 10–100 times cooler and denser than the surroundings and are often organized in larger events, termed showers. Thermal instability (TI) within a coronal loop in a state of thermal nonequilibrium (TNE) is the leading mechanism behind the formation of coronal rain but no investigation on showers exists to date. In this study, we conduct a morphological and thermodynamic multiwavelength study of coronal rain showers observed in an active region (AR) off-limb with IRIS and the Solar Dynamics Observatory, spanning chromospheric to transition region and coronal temperatures. Rain showers were found to be widespread across the AR over the 5.45 hr observing time, with an average length, width, and duration of 27.37 ± 11.95 Mm, 2.14 ± 0.74 Mm, and 35.22 ± 20.35 minutes, respectively. We find a good correspondence between showers and the cooling coronal structures consistent with the TNE–TI scenario, thereby properly identifying coronal loops in the “coronal veil”, including the strong expansion at low heights and an almost zero expansion in the corona. This agrees with previous work suggesting that the observed zero expansion in the EUV is due to specific cross-field temperature distribution. We estimate the total number of showers to be 155 ± 40, leading to a TNE volume of 4.56 ± [3.71] × 1028 cm3, i.e., on the same order of the AR volume. This suggests a prevalence of TNE over the AR indicating strongly stratified and high-frequency heating on average.

Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths

“…Rain clumps are produced in groups called showers (Antolin and Rouppe van der Voort, 2012) due to a syncing cross-field mechanism known as sympathetic cooling (Fang et al, 2013). These showers have been found to successfully identify coronal loops in the "coronal veil" produced by optically thin radiation (Sahin and Antolin, 2022;Malanushenko et al, 2022). The detection and quantification of rain showers is important for coronal heating since it is a proxy for the coronal volume in TNE (and thus subject to specific spatial and temporal heating conditions).…”

Decomposing the AIA~304 channel into its cool and hot components

“…The presence of coronal rain in the solar atmosphere is a strong proxy for strongly stratified, medium to high frequency heating in coronal structures (Antolin, 2020) or even coronal heating mechanisms (Antolin, Shibata, and Vissers, 2010). Its precise quantity at any given time is therefore an important variable to determine (S ¸ahin and Antolin, 2022). Several techniques have been developed that try to quantify the amount of plasma emitting in a given temperature range along a given line-of-sight (LOS).…”