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D.C. Braun (SPRC, HAO)
The application of helioseismic holography (also called seismic imaging) to solar data is among the latest developments in local helioseismology. This procedure applies helioseismic observations at the solar surface in time-reverse to a model of the solar interior, coherently regressing the surface disturbances downward to render acoustic images of subsurface structure. I will describe the basic concept, first developed ten years ago by C. Lindsey and D. Braun (1990 Solar Physics, 126, 101), and present new results from our recent application of the method to SOHO/MDI data. Notable discoveries include the ``acoustic moat'' (extended 3-mHz absorption surrounding all active regions), ``acoustic glories'' (high frequency halos of emission around complex active regions), acoustic condensations up to 20 Mm beneath active regions, and the 6-mHz acoustic reflectivity of active regions.
Recent results obtained with phase-sensitive seismic holography demonstrate a wave-speed perturbation beneath acoustic moats, which may be a manifestation of a localized convection cell predicted by theoretical models of sunspot convection and energy transport. The development of seismic-imaging procedures sensitive to Doppler and thermal perturbations promises to provide considerable insight into the physical conditions underlying active regions.
This research is supported by NSF Grants AST 9521637 and AST 9528249, and NASA Grants NAGW-97029 and NAG5-7236.
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