Rosseland Centre for Solar Physics, University of Oslo
Natural Sciences (Astrophysics and Astrononmy)
The chromosphere of the umbra of sunspots is a remarkably dynamic layer featuring extremely fine sub-arcsec structure. Such structures appear dark against enveloping umbral flashes, but also bright before or after a flash, other features still are bright throughout. Only recently did we start understanding such fine features and semi-empirical modelling is converging with simulations to provide insight, not only into such fine structure, but also into the umbral flash phenomenon itself. The observational evidence weighs overwhelmingly towards a strong corrugation of the umbra where the material in short dynamic fibrils over-extends in a column of upflowing material while the adjacent areas flash. The delayed small-scale umbral brightenings at the bottom of such columns are an out-of-phase
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