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

TL;DR

This study uses high-resolution IRIS data to detect supersonic downflows and heating events in the solar transition region above sunspots, revealing small-scale, dynamic phenomena linked to coronal rain and loop instability.

Contribution

It provides the first detailed observation of small-scale heating events and supersonic downflows in the transition region above sunspots using IRIS data.

Findings

Supersonic downflows of up to 200 km/s observed in transition region lines.

Bursty events last about 20 seconds and occur at sunspot footpoints.

Evidence of coronal rain and thermally unstable loops associated with these events.

Abstract

IRIS data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336 \AA, Si IV 1394 \AA, and 1403 \AA, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these short-lived events appear to be the umbral and penumbral footpoints of EUV loops. Fast apparent downflows are observed along these loops in the SJIs and in AIA, suggesting that the loops are thermally unstable. We interpret the observations as cool material falling from coronal heights, and especially coronal rain produced along the thermally unstable loops, which leads to an increase of intensity at the loop footpoints, probably indicating an increase of density and temperature in the TR. The rain speeds are on the higher end of previously reported speeds for this phenomenon, and possibly higher than the free-fall velocity along the loops. On other observing days, similar bright dots are sometimes aligned into ribbons, resembling small flare ribbons. These observations provide a first insight into small-scale heating events in sunspots in the TR.