Time-Distance Measurement of Solar Meridional Circulation and
Rotation From MDI 

P.M. Giles, T.L. Duvall, Jr.,
A.G. Kosovichev, and P.H. Scherrer

The emerging technique of time-distance helioseismology
can be used to measure acoustic wave travel times
between pairs of points on the solar surface.  The
measured time is sensitive to the properties of the
solar interior, including the flow of solar material
parallel to the direction of wave propagation.  Thus it
is possible to use the time-distance technique to
measure solar flows beneath the visible surface.

For example, by choosing pairs of points which share a
common longitude, it is possible to probe the depth
dependence of the meridional circulation.  Any flows
present will cause a difference between the northward
and southward travel times along the meridian.  Varying
the distance between points allows isolation of waves
which propagate to different depths beneath the surface,
and thus the flow velocity can be measured as a function
of latitude and depth.

Similarly, by choosing pairs of points which share a
common latitude it is possible to measure the effects of
solar rotation using an analogous procedure.  This
technique provides high resolution in latitude and
allows study of the northern and southern hemispheres
independently.

Using velocity images taken by the Michelson Doppler
Imager during solar minimum, we have detected a global
poleward flow in the uppermost layers of the sun.  We
also present measurements of the rotation velocity and
its north-south asymmetry as a function of depth.  This
research is supported by NASA contract NAG5-3077 at
Stanford University.