Evolution of supersonic downflows in a sunspot

¹ Astrophysics Research Centre (ARC), School of Mathematics and Physics, Queen’s University, Belfast BT7 1NN, UK
e-mail: c.nelson@qub.ac.uk
² Centre for mathematical Plasma Astrophysics (CmPA), KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium

Received: 19 December 2019
Accepted: 4 March 2020

Abstract

Context. Supersonic downflows have been observed in transition region spectra above numerous sunspots, however, little research has been conducted to date into how persistent these signatures are within sunspots on timescales longer than a few hours.

Aims. We aim to analyse the lead sunspot of AR 12526 to infer the properties and evolution of supersonic downflows occurring within it using high-spatial and spectral resolution data.

Methods. We analysed 16 large, dense raster scans sampled by the Interface Region Imaging Spectrograph. These rasters tracked the lead sunspot of AR 12526 across the solar disc at discrete times between 27 March 2016 and 2 April 2016, providing spectral profiles from the Si IV, O IV, Mg II, and C II lines. Additionally, we studied one sit-and-stare observation acquired on 1 April 2016 centred on the sunspot to analyse the evolution of supersonic downflows on shorter timescales.

Results. Supersonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. Thirteen of the 16 raster scans display some evidence of supersonic downflows in the Si IV 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel; a peak velocity of 112 km s⁻¹ is recorded on 29 March 2016. Evidence for supersonic downflows in the O IV 1401 Å line is found in 14 of these rasters; the spatial structuring in this line often differs from that inferred from the Si IV 1394 Å line. Only one example of a supersonic downflow is detected in the C II 1335 Å line and no downflows are found in the Mg II 2796 Å lines at these locations. In the sit-and-stare observations, no dual flow is initially detected, however, a supersonic downflow develops after approximately 60 min. This downflow accelerates from 73 km s⁻¹ to close to 80 km s⁻¹ in both the Si IV 1394 Å and O IV 1401 Å lines over the course of 20 min before the end of the observation.

Conclusions. Supersonic downflows are found in the Si IV 1394 Å line in 13 of the 16 rasters studied in this work. The morphology of these downflows evolved over the course of both hours and days and was often different in the Si IV 1394 Å and O IV 1401 Å lines. These events were found co-spatial to a bright region in the core of the Si IV 1394 Å line, which appeared to form at the footpoints of coronal fan loops. Our results indicate that one raster is not enough to conclusively draw inferences about the properties of supersonic downflows within a sunspot during its lifetime.