Title: Detection of Subsurface Supergranulation Structure and Flows
from MDI High-resolution Data using Time-Distance Techniques.

Authors: T.L. Duvall Jr.(NASA/GSFC), A.G. Kosovichev(Stanford), 
	P.H. Scherrer(Stanford), P.N.Milford(Parallel Rules, Inc.)

The supergranulation is seen at the surface of the sun in the
doppler shift of spectrum lines as an apparent cellular convection
pattern with a scale of about 4% of the solar radius.  This scale is
about 30 times larger than the granulation, seen in white light.
Why these distinct scales would be present (and possibly a third
intermediate scale mesogranulation) is somewhat of a mystery.  Also
unknown is the depth structure of the convection.

We have used acoustic wave measurements from the MDI experiment on
SOHO to address these questions.  By crosscorrelating the signal
at one location with that on annuli centered on the location, it is
possible to measure times for waves to travel over known subsurface
ray paths.  With some variations on this theme, it is possible to
measure horizontal and vertical flows and sound speed variations.
Of course, the resulting measurements refer to quantities integrated
along these ray paths.  An inversion technique based on Fermat's
principle has been developed and used to map the flow velocities
and sound speed variations as a function of horizontal position and
depth.  

The MDI experiment on SOHO makes doppler shift maps with 1Kx1K points
in two choices of image scale, 2 and 0.6 arcsec/pixel. 
For the present study, we have used the higher resolution mode to observe
8.5 hours of doppler maps sampled once per minute.  In order to average 
enough crosscorrelations to see time-distance effects, the resultant 
time-distance maps are reduced in resolution by a factor of 10 from 
the initial data.  This still yields about 7 samples across a single 
supergranulation cell, or 49 over the area of a square cell.

Our initial inversions based on the ray theory suggest that the 
supergranulation flow extends at least to 0.5% of the solar radius 
below the surface.

This research is supported by the SOI-MDI NASA contract NAG5-3077 at
Stanford University.