TIME-DISTANCE HELIOSEISMOLOGY WITH THE MDI INSTRUMENT - INITIAL RESULTS

In time-distance helioseismology, the travel time of acoustic waves is measured between various points on the solar surface. To some approxi mation, the waves can be considered to follow ray paths that depend on ly on a mean solar model, with the curvature of the ray paths being ca used by the increasing sound speed with depth below the surface. The t ravel time is affected by various inhomogeneities along the ray path, including flows, temperature inhomogeneities, and magnetic fields. By measuring a large number of times between different locations and usin g an inversion method, it is possible to construct 3-dimensional maps of the subsurface inhomogeneities. The SOI/MDI experiment on SOHO has several unique capabilities for time-distance helioseismology. The gre at stability of the images observed without benefit of an intervening atmosphere is quite striking. It has made it possible for us to detect the travel time for separations of points as small as 2.4 Mm in the h igh-resolution mode of MDI (0.6 are sec pixel(-1)). This has enabled t he detection of the supergranulation flow. Coupled with the inversion technique, we can now study the 3-dimensional evolution of the Rows ne ar the solar surface.