Subsurface structure of sunspots

The subsurface structure of sunspots is probed by measuring travel-time delays of acoustic waves propagating beneath the spots, and using a helioseismic inversion method to infer the internal sound-speed perturbations and mass flows. The initial results obtained from the MDI instrument on SOHO reveal under sunspots zones of the relatively low sound speed, extended to a depth of approximately 4 Mm. These zones are associated with cool areas of sunspots. In the deeper interior of sunspots the sound speed is higher than in the surrounding plasma. The regions of the higher sound speed are at least 60 Mm deep. These observations also show complicated flow patterns that include converging vortex flows in the upper 4 Mm deep layer. These flows play important role for maintaining the structure of sunspots. Developing active regions are associated with complicated evolving sound-speed perturbations beneath the surface, which are probably caused by multiple flux tubes emerging from the deep interior. A study of a rapidly rotating sunspot revealed strong shear flows beneath the spot.