Subphotospheric Structure of Sunspots and Active Regions

New methods of local area helioseismology provide three-dimensional maps of sound-speed variations and mass flows in the upper convection zone, giving important insight into the internal structure and dynamics of sunspots and active regions. Most of these results are obtained from SOHO/MDI data using the method of time-distance helioseismology (or acoustic tomography). Robustness of this method has been significantly improved by incorporating most important wave propagation effects and stochastic properties of solar oscillations. Time-distance helioseismology reveals that developed sunspots have a two layer structure: a relatively thin sub-photospheric layer of lower sound speed, and a deeper layer of higher sound speed. The mass flows in the upper layer are typically converging and directed downward, while in the deeper interior the flows are mostly diverging. These results support the cluster model of sunspots suggested by Parker. New observations also provide interesting information about emerging magnetic flux, formation and evolution of active regions and complexes of activity, and allow us to investigate effects of sub-photospheric dynamics of active regions on the global circulation of the Sun and also small-scale rapid shear flows associated with flares and CME.