Evolution Of Super-Sonic Downflows In A Sunspot

英文摘要：

Super-sonic 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 time-scales longer than a few hours. Here, we analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it. Sixteen large, dense raster scans sampled by IRIS are analysed. These rasters tracked the lead sunspot of AR 12526 at discrete times between the 27th March 2016 and the 2nd April 2016. One sit-and-stare observation acquired on the 1st April 2016 is also studied in order to analyse the evolution of super-sonic downflows on shorter time-scales. Super-sonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. 13 of the 16 raster scans display some evidence of super-sonic downflows in the Si IV 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel, with a peak velocity of 112 km s^{-1} being recorded on the 29th March 2016. Evidence for super-sonic downflows in the O IV 1401 Å line was found in 14 of these rasters, with the spatial structuring differing from that inferred from the Si IV 1394 Å line. In the sit-and-stare data, no dual flow is initially detected, however, a super-sonic downflow does develop after 60 minutes. 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 minutes before the end of the observation. 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 foot-points of coronal fan loops.