Compression, Impact and Hot Rebound Flows from Coronal Rain Downflows

TL;DR

This study analyzes a quiescent coronal rain event using multi-instrument, high-resolution observations to quantify how rain clumps modulate loop dynamics and heating. By tracing rain trajectories with cubic splines, performing DEM-based thermodynamics, and triangulating speeds from multiple vantage points, the authors show that rain-induced compression is largely isothermal and leads to hot rebound flows that refill and reheat the loop, all within a thermal-non-equilibrium/thermal-instability framework. The energy analysis reveals a microflare-level shower energy ($\sim 4.64\times10^{26}$ erg) with roughly 15% of clump kinetic energy transferred to rebound flows, while the majority of energy is radiated in the transition region, supporting accretion-braking as a mechanism in loop dynamics. Overall, coronal rain emerges as a practical probe of coronal heating and energy transport in TNE-TI cycles, with implications for heating scale heights and persistent footpoint heating in active regions.

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 Å. 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.

Figures (20)

• Figure 1: The location of SolO and SDO in relation to the Sun and Earth on November 1st, 2023, obtained from the Propagation Tool software rouillard2017propagation.
• Figure 2: The active region observed in AIA showing the SJI 1400 Å FOV and the sub-FOV shown in Figure \ref{['fig:171_sub_fov']}. The rain trajectory is plotted in white as 'original spline'
• Figure 3: The studied AR on the solar disc with HRIEUV. The sub-FOV enclosed in green is shown in Figure \ref{['fig:hri_sub_fov']}. The rain trajectory is plotted in white and is denoted as the 'original spline'.
• Figure 4: The coronal loop along which the coronal rain is seen to fall with HRIEUV. The 'Original Spline' corresponds to the initial coordinates used to trace the rain trajectory. Spline$\pm x$ is a shift of the original spline by $x$ pixels along the $x$-axis, created to fully capture the extent of the rain shower. 'Path 1' is used to assess the EUV variability in the loop, and 'Path 2' is used to estimate the FWHM (Figure \ref{['fig:hri_td_paths']}). The 'Impact Region' is where the rain is observed to impact the lower atmosphere. We plot the intensity in logarithmic scale to make the coronal loop more visible.
• Figure 5: The coronal loop along which the coronal rain is seen to fall in AIA. Same as in Figure \ref{['fig:hri_sub_fov']} but for AIA 171 Å.