Compression, Impact and Hot Rebound Flows from Coronal Rain Downflows

Abstract

Studying coronal rain formation through thermal non-equilibrium (TNE) and thermal instability (TI) provides insights into coronal heating mechanisms. We analysed a quiescent coronal rain event using space-based observations from the High-Resolution Imager in Extreme Ultraviolet (\hrieuv) of Solar Orbiter (SolO), the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO), and the Slit-Jaw Imager (SJI) from the Interface Region Imaging Spectrograph (IRIS) from November 1st, 2023. During the coronal rain shower, the coronal loop exhibits substantial EUV variability and structural changes. Rain clumps fell at $72-87$ km s$^{-1}$ with cool EUV absorbing core sizes of $\approx$600 km and densities of $\approx6\times10^{11}$ cm$^{-3}$ preceded by strong compressions. These mostly isothermal compressions suggest energy transfer into the rain, decelerating it and possibly reducing cooling rates -- consistent with accretion braking timescales. The shower carried microflare-level energy ($4.64\times10^{26}$ erg), with clumps producing impacts that reach the lower transition region and are visible across all EUV channels and in SJI 1400 \AA. The impacts generated hot rebound flows ($10^{6.2}-10^{6.3} $K, $85-87$ km s$^{-1}$) that refilled and reheated the loop but carried less than $15\%$ of the clumps' kinetic energy. We detected steady footpoint heating signatures consistent with the TNE-TI scenario, with an estimated amplitude of $10^{-2\pm0.3}$ erg cm$^{-3}$ s$^{-1}$ and heating scale heights of $2-10$~Mm, matching active region values. Coronal rain may thus serve as both a template for accretion braking and a proxy for integrated heating driving TNE-TI cycles.