Properties of granular light bridges

Abstract

Granular light bridges in sunspots are a manifestation of magnetoconvection in an approximately vertical magnetic field that is weaker than the field of the umbra. We study the relations between observed properties of granules and magnetic field in five strong granular light bridges located in a complex sunspot. Thermal parameters, magnetic field vector, and line-of-sight velocity were obtained from an inversion of the GREGOR spectropolarimetric scan in the lines Si I 1082.71 nm and Ca I 1083.90 nm. Sizes and lifetimes of granules were measured in a simultaneous sequence of images in the TiO band. The light bridges under study are comprised of granules spanning 0.2"-1.4" with approximately 6-minute lifetimes. The largest granular structures (> 1") are spatially correlated with 1 km/s upflows and weak, inclined magnetic fields (300 G) near the equipartition threshold. Statistical comparisons demonstrate an inverse relationship between magnetic field strength and continuum intensities, line-of-sight velocity dispersion, and effective granular diameters. These relationships vary among individual light bridges, depending on the large-scale magnetic field topology. Large expanding granules similar to the photospheric ones appear in light bridges only when the local magnetic field strength approaches the equipartition value.