Statistical Investigation of Supersonic Downflows in the Transition Region above Sunspots

Abstract

Downflows at supersonic speeds have been observed in the transition region (TR) above sunspots for more than three decades. These downflows are often seen in different TR spectral lines above sunspots. We have performed a statistical investigation of these downflows using a large sample which was missing earlier. The Interface Region Imaging Spectrograph (IRIS) has provided a wealth of observational data of sunspots at high spatial and spectral resolution in the past few years. We have identified sixty datasets obtained with IRIS raster scans. Using an automated code, we identified the locations of strong downflows within these sunspots. We found that around eighty percent of our sample show supersonic downflows in the Si IV 1403 Å line. These downflows mostly appear in the penumbral regions, though some of them are found in the umbrae. We also found that almost half of these downflows show signatures in chromospheric lines. Furthermore, a detailed spectral analysis was performed by selecting a small spectral window containing the O IV 1400/1401 Å and Si IV 1403 Å lines. Six Gaussian functions were simultaneously fitted to these three spectral lines and their satellite lines associated with the supersonic downflows. We calculated the intensity, Doppler velocity and line width for these lines. Using the O IV 1400/1401 Å line ratio, we find that the downflow components are around one order of magnitude less dense than the regular components. Results from our statistical analysis suggest that these downflows may originate from the corona and that they are independent of the background TR plasma.

13 pages, 10 figures, Accepted for publication in the ApJ

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• Coronal Condensation as the Source of Transition Region Supersonic Downflows above a Sunspot
• Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes
• Umbral chromospheric fine structure and umbral flashes modelled as one: the corrugated umbra
• Evolution Of Super-Sonic Downflows In A Sunspot
• Magnetized supersonic downflows in the chromosphere. A statistical study using the He I 10830 Å lines
• On the Response of the Transition Region and the Corona to Rapid Excursions in the Chromosphere
• Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)