Observing the Formation of Flare-Driven Coronal Rain

Abstract

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×10 11 ±1.76×10 11 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×10 12 ±4.95×10 11 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.

High-density Off-limb Flare Loops Observed by SDO

“…An analogous analysis method was developed for solar prominences (Jejčič & Heinzel 2009), which also represent cool off-limb structures (note that flare loops have been previously classified as 'loop prominences', but now they are often called 'coronal rain',see e.g. Scullion et al 2016), however, the electron densities of prominences are low and thus the Thomson scattering completely dominates their WL emission, which can be detected only during solar eclipses. In Figure 5 n e = 10 11 cm −3 refers to an upper limit of electron density usually met in quiescent prominences and the continuum intensity is thus a few orders of magnitude lower than that detected in our studied flare loops.…”

Coronal rain in magnetic bipolar weak fields

“…The electron density of rain clumps was found to be about 1.8-7.1×10 10 cm −3 , through estimation based on absorption in multiple extreme ultraviolet (EUV) channels. Besides this quiescent coronal rain, which occurs in nonflaring coronal loops with relatively weak variation of energy and mass, flare-driven coronal rain, which appears in postflare loops as a result of catastrophic cooling, often emerges as a bunch of parallel strands extending from loop top to footpoint (Scullion et al 2014(Scullion et al , 2016.…”

The Coronal Monsoon: Thermal Nonequilibrium Revealed by Periodic Coronal Rain

“…Any additional mass then initiates runaway cooling and the ensuing condensation-the "rain"-rapidly grows to ultimately fall into the chromosphere, thus draining the loop down to its initial lower density. In the flaredriven scenario, the rain is triggered in otherwise quiescent loops by a single and long enough burst of intense footpoint heating (Scullion et al 2016). But if both the geometry and the heating are steady, the process repeats itself over and over again.…”