Recent helioseismic images of a sunspot computed by Charlie Lindsey (SPRC) and Doug Braun (HAO) show a remarkable acoustic anomaly surrounding a sunspot. Lindsey and Braun applied a computational technique they call "acoustic holography" to SoHO-MDI observations of NOAA Active Region 7973, which contains a moderately large sunspot, over a 24-hour period beginning on 1996 June 25.0. The resulting acoustic images show a conspicuous halo 70,000 km in diameter surrounding the sunspot in which there appears a predominant acoustic deficit. This "acoustic moat" is terminated by a sharp outer boundary completely circumscribing the sunspot. The outer boundary of the acoustic moat coincides strongly with plages in the neighborhood of the sunspot.
Lindsey and Braun believe that the acoustic moat is the helioseismic signature of a rapid outflow surrounding the sunspot not far beneath the solar surface. The fact that sunspots absorb acoustic waves was discovered by Braun and colleagues in the 1980s. The acoustic moat appears to be the result of Doppler scattering, by the surrounding rapid outflow, of the acoustic deficit induced by the sunspot.
Depth analysis, based on focus-defocus diagnostics of the image coherently extrapolated to focal planes at various subsurface levels, show the acoustic perturbations signifying both the sunspot and the acoustic moat to be predominantly superficial, existing within a few thousand km of the solar surface. This is shown in the figure, in which the image rendered at the surface (Frame a) rapidly defocuses as the focal plane is submerged to 5,600 km, 8,400 km and 11,200 km, in Frames b, c, and d, respectively. The inset at the center of the figure shows the line-of-sight magnetic field taken by the spectromagnetograph at NSO's Kitt Peak Vacuum Telescope on 1996 June 25. A close comparison of the inset with Frame a of the figure shows that plages in the south and east quadrants of the sunspot correspond to the boundary of the acoustic moat.
Following theoretical work in the 1970s by F. Meyer and colleagues, Lindsey and Braun propose that that acoustic moat is the helioseismic manifestation of a single, integrated convection eddy that is driven by heat accumulation resulting from the local blockage of convective transport from the solar interior into the sunspot subphotosphere. With the advent of SoHO and GONG, helioseismic holography promises considerable insight into convective flows surrounding sunspots, an issue that is certain to be critical to the long standing problem of thermal transport in the neighborhoods of sunspots.