  Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with
speeds of 100 km/s. These downflows are supersonic at transition region
temperatures of 0.1 MK. The source of these flows is not well understood. We
aim to investigate the source of sunspot supersonic downflows (SSDs) in AR
12740 using simultaneous spectroscopic and imaging observations. We identified
SSD events from multiple raster scans of a sunspot by the Interface Region
Imaging Spectrograph, and calculated the electron densities, mass fluxes and
velocities of these SSDs. The EUV images provided by the AIA onboard the SDO
and the EUVI onboard the STEREO were employed to investigate the origin of
these SSDs and their associated coronal rain. Almost all the identified SSDs
appear at the footpoints of sunspot plumes and are temporally associated with
appearance of chromospheric bright dots inside the sunspot umbra.
Dual-perspective EUV imaging observations reveal a large-scale closed magnetic
loop system spanning the sunspot region and a remote region. We observed that
the SSDs are caused by repeated coronal rain that forms and flows along these
closed magnetic loops toward the sunspot. One episode of coronal rain clearly
indicates that reconnection near a coronal X-shaped structure first leads to
the formation of a magnetic dip. Subsequently, hot coronal plasma
catastrophically cools from 2 MK in the dip region via thermal instability.
This results in the formation of a transient prominence in the dip, from which
the cool gas mostly slides into the sunspot along inclined magnetic fields
under the gravity. This drainage process manifests as a continuous rain flow,
which lasts for around 2 hrs and concurrently results in a nearly steady SSD
event. Our results demonstrate that coronal condensation in magnetic dips can
result in the quasi-steady sunspot supersonic downflows.
