Umbral and penumbral magnetoconvection

Compressible convection in a strong vertical magnetic field can take two forms: either there is a regular pattern of small plumes or else the field separates from the motion giving regions of vigorous convection and regions that are magnetically dominated, In an inclined field, patterns are bound to travel. When the magnetic field is almost horizontal, convection takes the form of rolls aligned with the field. In the umbra of a sunspot convection will take the form of spatially modulated oscillations, giving rise to umbral dots. The penumbral field shows an interlocking-comb structure, In the bright filaments the magnetic field is inclined to the horizontal and modulated travelling waves, moving inwards or outwards, can give rise to the migration of bright grains. In the dark filaments the field is nearly horizontal, allowing only inefficient overturning convection.