Context : 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 .

Aims : Here , we aim to analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it using high-spatial and spectral resolution data .

Methods : Sixteen large , dense raster scans sampled by the Interface Region Imaging Spectrograph are analysed .
These rasters tracked the lead sunspot of AR 12526 across the solar disc at discrete times between the 27 th March 2016 and the 2 nd April 2016 , providing spectral profiles from the Si IV , O IV , Mg II , and C II lines .
Additionally , one sit-and-stare observation acquired on the 1 st April 2016 centred on the sunspot is studied in order to analyse the evolution of super-sonic downflows on shorter time-scales .

Results : 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 29 th 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 in this line often differing from that inferred from the Si IV 1394 Å line .
Only one example of a super-sonic downflow was detected in the C II 1335 Å line , with no downflows being 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 super-sonic downflow does develop after around 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 .

Conclusions : Super-sonic downflows were found in the Si IV 1394 Å line in 13 of the 16 rasters studied here .
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 .
Our results indicate that one raster is not enough to conclusively draw inferences about the properties of super-sonic downflows within a sunspot during its lifetime .