An explanation of remarkable emission line profiles in post-flare coronal rain

Abstract

We study broad red-shifted emission in chromospheric and transition region lines that appears to correspond to a form of post-flare coronal rain. Profiles of Mg II, C II and Si IV lines were obtained using the IRIS instrument before, during and after the X2.1 flare of 11 March 2015 (SOL2015-03- 11T16:22). We analyze the profiles of the five transitions of Mg II (the 3p - 3s h and k transitions, and three lines belonging to the 3d - 3p transitions). We use analytical methods to understand the unusual profiles, together with higher resolution observational data of similar phenomena observed by Jing et al. (2016). The peculiar line ratios indicate anisotropic emission from the strands which have cross-strand line center optical depths (k-line) of between 1 and 10. The lines are broadened by unresolved Alfvenic motions whose energy exceeds the radiation losses in the Mg II lines by an order of magnitude. The decay of the line widths is accompanied by a decay in the brightness, suggesting a causal connection. If the plasma is <~ 99% ionized, ion-neutral collisions can account for the dissipation, otherwise a of dynamical process seems necessary. Our work implies that the motions are initiated during the impulsive phase, to be dissipated as radiation over a period of an hour, predominantly by strong chromospheric lines. The coronal "rain" we observe is far more turbulent that most earlier reports have indicated, with implications for plasma heating mechanisms.