TY - JOUR
T1 - Prevalence of Thermal Nonequilibrium over an Active Region
AU - Sahin, Seray
AU - Antolin, Patrick
N1 - Funding information: The authors would like to thank the anonymous referee for the constructive comments and acknowledge T. Schad for his help with the RHT algorithm. P.A. acknowledges funding from his STFC Ernest Rutherford Fellowship (No. ST/R004285/2). This study used openly available IRIS and SDO data at https://www.lmsal.com. IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research Center and major contributions to downlink communications funded by ESA and the Norwegian Space Centre. SDO is part of NASA's Living With a Star Program.
PY - 2022/6/1
Y1 - 2022/6/1
N2 - 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.
AB - 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.
KW - 360
KW - The Sun and the Heliosphere
UR - http://www.scopus.com/inward/record.url?scp=85131396448&partnerID=8YFLogxK
U2 - 10.3847/2041-8213/ac6fe9
DO - 10.3847/2041-8213/ac6fe9
M3 - Article
SN - 2041-8205
VL - 931
JO - The Astrophysical Journal Letters
JF - The Astrophysical Journal Letters
IS - 2
M1 - L27
ER -