  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.
