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Session 79 - Quiet Solar Photosphere and Chromosphere.
Oral session, Thursday, June 13
We used the Bartol-NSO-NASA South Pole helioseismic observations of 1991 January to study the subsurface structure of active regions to depths of \sim 15,000 km. The helioseismic signature we particularly examine is based on acoustic Doppler effects caused by horizontal flows associated with the active region. We demonstrate that the Doppler-acoustic signature of horizontal flows is particularly well suited for deep subsurface diagnostics in terms of vertical discrimination of the structure. This study is based primarily on observations of NOAA active regions 6431, 6432, 6440 and 6442 between 1991 January 1 and January 8. We interpret the Doppler signatures we find in terms of a general outflow of the solar medium surrounding the active region. The existence of deep subsurface structure is indicated by the strong dependence of the Doppler signature on horizontal wavelength. The outflows in surface layers, the upper 4,000 km of the subphotosphere, are quite weak but increase strongly with depth to velocities of several hundred m/s at 15,000 km. This depth profile evolves rapidly as the active region matures. Young active regions show strong outflows at depths between 4,000 and 8,000 km. As the active region matures, the outflow vacates these intermediate layers and submerges to depths mostly below 8,000 km.
We examine the location of Region 6442 for a possible pre-emergence signature. We also show strong evidence for extended, relatively superficial flows in the quiet Sun between the active-region bands directed roughly into the active region bands.
Program listing for Thursday