Seismic Tomography of the Near Solar Surface

Abstract. Surface gravity waves have been used to probe flows in the two megameters beneath the photosphere using the techniques of time distance helioseismology. The results suggest that supergranule veloci ties are smaller than at the surface. The outward flow outside a sunspot penumbra (the moat) is observed, as is an inward flow in the region beyond the moat. Key words. Helioseismology—supergranulation—sunspots. 1. Timedistance helioseismology with surface gravity waves Local helioseismology diagnostics open the prospect of mapping the threedimen sional structure and dynamics of the upper solar convection zone. Duvall & Gizon (2000) recently applied timedistance helioseismology to surface gravity waves, to probe horizontal flows in the first two megameters below the photosphere. Promising results were obtained using a simple calibration instead of a proper inversion. Here we have analyzed an 8hour long time series of dopplergrams observed on 1998 December 6 in the MDI highresolution field of view (Scherrer et al. 1995). Rotation was removed. The datacube was Fourier transformed in time and space, and acoustic waves filtered out. The temporal cross correlations were computed and averaged using the quadrant geometry, as described by Duvall & Gizon (2000). For each pixel location, x, we obtained three crosscorrelation functions, denoted CEW, CSN and CIO, which depend on correlation time τ and distance from x. They respectively contain local information about the westward component, northward component, and divergence of the flow. A fourth crosscorrelation function, C0, was constructed by averaging over all possible pairs of points; it is symmetric with respect to correlation time. Flows introduce a correlationtime asymmetry in CEW, CSN and CIO, which we measure by cross correlation with the template C0. We finally get timeasymmetry maps δ τ E W, δτ S N, and δτI0 for each distance. Further averaging over in the range 4.5–11.9 Mm provides us with three maps, labeled