Observation and Analysis of Ballistic Downflows in an M-class Flare with the Interface Region Imaging Spectrograph

Abstract

Despite significant advances in instrumentation, there remain no studies that analyze observations of on-disk flare loop plasma flows covering the entire evolution from chromospheric evaporation, through plasma cooling, to draining downflows. We present results from an imaging and spectroscopic observation from the Interface Region Imaging Spectrograph (IRIS) of the SOL2015-03-12T11:50:00 M-class flare, at high spatial resolution and time cadence. Our analysis of this event reveals initial plasma evaporation at flare temperatures indicated by 100-200 km s^-1 blueshifts in the Fe xxi line. We subsequently observe plasma cooling into chromospheric lines (Si IV and O IV) with ∼11 minute delay, followed by loop draining at ∼40 km s^-1 as indicated by a “C”-shaped redshift structure and significant (∼60 km s^-1) non-thermal broadening. We use density-sensitive lines to calculate a plasma density for the flare loops, and estimate a theoretical cooling time approximately equal to the observed delay. Finally, we use a simple elliptical free-fall draining model to construct synthetic spectra, and perform what we believe to be the first direct comparison of such synthetic spectra to observations of draining downflows in flare loops.