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D.C. Braun (SPRC, HAO), C. Lindsey (SPRC)
Phase-correlation statistics comparing acoustic radiation coming out of a particular point on the solar photosphere with acoustic radiation going into it show considerably reduced sound travel times through the subphotospheres of active regions. This is already well established by time-distance correlations measured by Duvall et al., and is consistent with earlier measurements of scattering phase shifts of single sunspots by Braun and Fan. We have now applied techniques in phase-sensitive seismic holography to obtain high resolution phase-correlation maps of active regions and the ``acoustic moats'' that surround them. The important new result which the holographic correlation maps give us is that the seismic perturbation manifested by the acoustic moats is generally quite significant and in large active-region complexes predominant. Indeed, the acoustic moat surrounding the large active region complex NOAA~AR~8179 (1998 March 16) manifests a one-way travel-time reduction of ~~30s over an area of some 104 Mm2, encompassing all of the significant sunspots in the region. Onto this phase perturbation the major sunspots impose an additional localized contribution of ~~30s. These results strongly reinforce an interpretation of the acoustic moat as a well integrated convection cell driven by the thermal perturbation that results from the local blockage of convective heat transport by the sunspot photosphere. The remarkable lateral extent of the acoustic moat, some 100~Mm in latitude and more than 150~Mm in longitude, suggests a convective flow that undertakes to spread the thermal perturbation into a relatively thin layer, such that the excess heat contained therein can access the solar surface through normal supergranular and granular diffusion. We expect holographic Doppler diagnostics, now under development, to shed considerable light very soon on the flows that are needed to explain the extended dimensions of the thermal perturbations that surround large 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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