Strong, Localized Downflows in a Sunspot Light Bridge

Abstract

We investigate spectropolarimetric observations of a sunspot light bridge in NOAA AR 10953 taken on 1 May 2007 with Hinode using the Fe I line pair at 630 nm. The SIR code (Ruiz Cobo and del Toro Iniesta 1992, ApJ, 398, 375) was used to invert the observed Stokes profiles, obtaining temperature stratifications and height-independent values of the magnetic field and Doppler velocity. The maps of the physical parameters show that the light bridge is a penumbral penetration into the umbra and has a relatively weak, inclined magnetic field. The highlight of our inversions is the presence of strong downflow patches in the light bridge, with line-of-sight velocities exceeding 4 km s−1. The field azimuth also shows large rotation along a thin ridge close to one edge of the light bridge, essentially seen as a discontinuity in azimuth. Some of the downflow patches are also co-spatial with brightness enhancements in the Ca IIH chromospheric filtergrams (Louis et al. 2008, Solar Phys., 252, 43). Inspection of the Stokes profiles for the downflow patches indicates doubly red-lobed Stokes-V signals. These profiles were also inverted with SIR using a two-component atmosphere with varying degrees of complexity. All such inversions indicate that the downflow patches consist of supersonic flows of about 10 km s−1. Interestingly, the linear polarization also appears to be anomalous at the ridge demarcating the field azimuth change.We believe that the anomalous Q and U profiles result from mixing of the light bridge and the umbral magnetic field, which through reconnection may result in supersonic downflows in the photosphere and brightness enhancements in the chromosphere. The light bridge represents a strong inhomogeneity within a fairly regular sunspot. Our observations serve as useful inputs to future numerical models of light bridges.