Cold, supersonic Evershed downflows in a sunspot

We report here on the discovery of supersonic Evershed downflows in the pen umbra of a sunspot. These flows are shown to occur along spatially unresolv ed, very cold magnetic flux tubes whose downflowing footpoints are found fr om the middle penumbra outward. Evershed flows along magnetic field lines r eturning to the solar surface were discovered by Westendorp Plaza and cowor kers, but only in the outer parts of the penumbra and beyond its visible bo undary; on the other hand, no supersonic flows of any type have ever been r eported in the photosphere of sunspots, except for the very different case of the delta spot analyzed by Martinez Pillet and coworkers. We present une quivocal evidence of such supersonic motions, already predicted theoretical ly by the siphon-flow model, from the interpretation of infrared spectropol arimetric observations of a sunspot with unprecedented spatial resolution.