Observing the formation of flare-driven coronal rain

Flare-driven coronal rain can manifest from rapidly cooled plasma condensations near coronal loop tops in thermally unstable postflare arcades. We detect five phases that characterize the postflare decay: heating, evaporation, conductive cooling dominance for ~120 s, radiative/enthalpy cooling dominance for ~4700 s, and finally catastrophic cooling occurring within 35–124 s, leading to rain strands with a periodicity of 55–70 s. We find an excellent agreement between the observations and model predictions of the dominant cooling timescales and the onset of catastrophic cooling. At the rain-formation site, we detect comoving, multithermal rain clumps that undergo catastrophic cooling from ~1 MK to ~22,000 K. During catastrophic cooling, the plasma cools at a maximum rate of 22,700 K s−1 in multiple loop-top sources. We calculated the density of the extreme-ultraviolet (EUV) plasma from the differential emission measure of the multithermal source employing regularized inversion. Assuming a pressure balance, we estimate the density of the chromospheric component of rain to be 9.21 × 10¹¹ ± 1.76 × 10¹¹ cm⁻³, which is comparable with quiescent coronal rain densities. With up to eight parallel strands in the EUV loop cross section, we calculate the mass loss rate from the postflare arcade to be as much as 1.98 × 10¹² ± 4.95 × 10¹¹ g s⁻¹. Finally, we reveal a close proximity between the model predictions of 10^5.8 K and the observed properties between 10^5.9 and 10^6.2 K, which defines the temperature onset of catastrophic cooling. The close correspondence between the observations and numerical models suggests that indeed acoustic waves (with a sound travel time of 68 s) could play an important role in redistributing energy and sustaining the enthalpy-based radiative cooling.