The Acoustic Moat and Thermal Transport in the Neighborhoods of Sunspots

Abstract

Helioseismic holography of sunspots shows a remarkable acoustic anomaly surrounding the sunspot, which we call an "acoustic moat." We used Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager observations of NOAA Active Region 7973 to render acoustic images of a typical sunspot. These images show a conspicuous halo, 70,000 km in diameter, surrounding the sunspot, in which there appears a predominant acoustic deficit. This "acoustic moat" may be terminated by a sharp outer boundary, which appears to circumscribe the sunspot completely in some instances. The outer boundary of the acoustic moat coincides conspicuously with plages in the neighborhood of the sunspot. Depth diagnostics based on acoustic focus suggest that the acoustic perturbations characterizing both the sunspot and the acoustic are predominantly superficial, within a few thousand kilometers of the solar surface. Following work by Meyer et al., we propose that the 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. We propose that the acoustic deficit predominantly characterizing the halo is the result of thermal refraction or Doppler scattering by the eddy outflow of an acoustic deficit originating in the helioseismic absorption by the nearby sunspot and possibly neighboring plages. With the advent of SOHO and the Global Oscillation Network Group, helioseismic holography promises considerable insight into the general subject of 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.