The Coronal Monsoon: Thermal Nonequilibrium Revealed by Periodic Coronal Rain

TL;DR

This paper reveals that periodic coronal rain and loop intensity pulsations are interconnected phenomena caused by thermal nonequilibrium, providing a unified understanding of coronal rain and loop dynamics observed by SDO/AIA.

Contribution

It demonstrates that coronal rain and loop pulsations are manifestations of the same thermal nonequilibrium process, unifying previous observations and models.

Findings

Coronal rain occurs periodically with a 6.6-hour cycle.

Loop intensity pulsations are synchronized with coronal rain.

Thermal nonequilibrium explains both phenomena.

Abstract

We report on the discovery of periodic coronal rain in an off-limb sequence of {\it Solar Dynamics Observatory}/Atmospheric Imaging Assembly images. The showers are co-spatial and in phase with periodic (6.6~hr) intensity pulsations of coronal loops of the sort described by Auchere et al. (2014) and Froment et al. (2015, 2017). These new observations make possible a unified description of both phenomena. Coronal rain and periodic intensity pulsations of loops are two manifestations of the same physical process: evaporation / condensation cycles resulting from a state of thermal nonequilibrium (TNE). The fluctuations around coronal temperatures produce the intensity pulsations of loops, and rain falls along their legs if thermal runaway cools the periodic condensations down and below transition-region (TR) temperatures. This scenario is in line with the predictions of numerical models of quasi-steadily and footpoint heated loops. The presence of coronal rain -- albeit non-periodic -- in several other structures within the studied field of view implies that this type of heating is at play on a large scale.