Probing magnetic structures in the solar interior by helioseismic tomography

Helioseismic tomography provides a unique opportunity to probe magnetic field structures and dynamics in the solar interior. This technique (also known as time-distance helioseismology) is based on measurements of travel times of acoustic waves propagating through the interior. Variations of the travel times are related to variations of the sound speed, magnetic anisotropies, and mass flows. By combining travel time measurements for waves propagating in different directions and using an inversion procedure it is possible to disentangle these effects and construct 3-D maps of internal structures and flows associated with magnetic fields. We present some results of imaging of emerging active regions and sunspots using data from the MDI instrument of the SOHO space mission. The initial results demonstrate that the emerging active regions travel very quickly through the upper 20 Mm of the convection zone. The estimated speed of emergence, similar to1.3 km/s, is higher than the speed predicted by theories. The internal structure of sunspots shows two zones: a zone of a reduced wave speed (lower temperature) similar to4 Mm deep, and a zone of a higher wave speed in the deeper layers. Mass flows around a sunspot converge beneath the spot in agreement with Parker's cluster sunspot model.