Downflows in sunspot umbral dots

Abstract

We study the velocity field of umbral dots (UDs) at a resolution of 0. ″14. Our analysis is based on full Stokes measurements of a pore taken with the Crisp Imaging Spectro-Polarimeter at the Swedish 1 m Solar Telescope. We determine the flow velocity at different heights in the photosphere from a bisector analysis of the Fe I 630nm lines. In addition, we use the observed Stokes Q, U, and V profiles to characterize the magnetic properties of these structures. We find that most UDs are associated with strong upflows in deep photospheric layers. Some of them also show concentrated patches of downflows at their edges, with sizes of about 0.″25, velocities of up to 1000ms -1, and enhanced net circular polarization signals. The downflows evolve rapidly and have lifetimes of only a few minutes. These results appear to validate numerical models of magnetoconvection in the presence of strong magnetic fields. © 2010. The American Astronomical Society. All rights reserved.