Evidence for Magnetoconvection in a Sunspot Light Bridge

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Abstract
We study a time sequence of the velocity field measured in a sunspot light bridge classified as photospheric. The data provide clear evidence for a convective origin of the photospheric light-bridge phenomenon. Furthermore, the data strongly indicate that a form of oscillatory magnetoconvection is the physical process responsible for the formation of the observed light bridge. We observe umbral dots and find in them a very small upward velocity of order 50 m s-1. Both the intensity and the velocity signal vary in time. Some umbral dots fade over a period of about 15-20 minutes and then reappear at the same location. In the particular sunspot observed, umbral dots give the impression of being aligned in lanes that appear to separate individual flux fragments within the umbra.