Observations of Supersonic Downflows Near the Umbra-Penumbra Boundary of Sunspots as Revealed by Hinode

High resolution spectropolarimetric observations by Hinode reveal the existence of supersonic downflows at the umbra-penumbra boundary of 3 sunspots that was recently reported by Louis et al. (2010). These downflows are observed to be co-spatial with bright penumbral filaments and occupy an area greater than 1.6 arcsec$^2$. They are located at the center-side penumbra and have the same polarity as the sunspot which suggests that they are not associated with the Evershed Flow. In this paper we describe the supersonic velocities observed in NOAA AR 10923 and discuss the photospheric as well as chromospheric brightenings, that lie close to the downflowing areas. Our observations suggest that this phenomenon is driven by dynamic and energetic physical processes in the inner penumbra which affect the overlying chromosphere and thus provide new insights for numerical models of sunspots.