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IRIS data allow us to study the solar transition region (TR) with anunprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observedbursts of high Doppler shifts suggesting strong supersonic downflows of up to200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h andk, C II 1336 \AA, Si IV 1394 \AA, and 1403 \AA, that are correlated withbrightenings in the slitjaw images (SJIs). The bursty behavior lasts throughoutthe 2 hr observation, with average burst durations of about 20 s. The locationsof these short-lived events appear to be the umbral and penumbral footpoints ofEUV loops. Fast apparent downflows are observed along these loops in the SJIsand in AIA, suggesting that the loops are thermally unstable. We interpret theobservations as cool material falling from coronal heights, and especiallycoronal rain produced along the thermally unstable loops, which leads to anincrease of intensity at the loop footpoints, probably indicating an increaseof density and temperature in the TR. The rain speeds are on the higher end ofpreviously reported speeds for this phenomenon, and possibly higher than thefree-fall velocity along the loops. On other observing days, similar brightdots are sometimes aligned into ribbons, resembling small flare ribbons. Theseobservations provide a first insight into small-scale heating events insunspots in the TR.

Detection of supersonic downflows and associated heating events in the transition region above sunspots