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

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 Å, Si IV 1394 Å, and 1403 Å, 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.

arXiv commentaccepted by ApJL

References in corpus (13)

• The Interface Region Imaging Spectrograph (IRIS)
• The Dynamic Formation of Prominence Condensations
• Observing the fine structure of loops through high resolution spectroscopic observations of coronal rain with the CRISP instrument at the Swedish Solar Telescope
• Coronal rain as a marker for coronal heating mechanisms
• High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph
• Observing coronal nanoflares in active region moss
• Multidimensional modeling of coronal rain dynamics
• Transverse oscillations of loops with coronal rain observed by Hinode/SOT
• Solar transition region above sunspots
• Sparkling EUV bright dots observed with Hi-C
• Prominence plasma diagnostics through EUV absorption
• Spectropolarimetry of C-class flare footpoints
• Post-flare UV light curves explained with thermal instability of loop plasma

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