Helioseismic Measurements of Subsurface Meridional Flow from MDI
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P M Giles, T L Duvall, Jr, and P H Scherrer


The meridional circulation is expected to play an important
role in solar dynamics.  This flow has previously been
observed at the solar surface in Doppler velocity images,
and through tracking of magnetic features.  Recently, we
have used time-distance helioseismology to study the depth
dependence of the flow.  We have observed a poleward flow,
with a maximum velocity of about 20 m/s, in the upper layers
of the solar convection zone.  The flow persists to the
maximum depth of our observations, which is roughly 5
percent of the solar radius.

These recent investigations, using the Michelson Doppler
Imager on the SOHO spacecraft, have made use of the MDI
full-disk dynamics program (2 arc-sec/pixel).  The analysis
is computationally intensive, and due to the high resolution
of these images, it has proven difficult to extend the
region of study very far into the solar interior.
Furthermore, the dynamics program runs for only a few months
each year, making it unsuitable for studies of the time
dependence of the flows.

It should be possible to use the MDI full-disk structure
program (10 arc-sec/pixel) to extend this research.  The
structure program runs continuously, which will allow study
of the solar-cycle dependence of the flows as the sun's
activity increases.  It will also allow us to look for
longitudinal structure in the meridional circulation.
Furthermore, lower spatial resolution of the images should
allow for a probing of deeper depths.

We will present results which extend our previous
measurements of the solar meridional flow as a function of
depth and latitude.  This research is supported by NASA
contract NAG5-3077 at Stanford University.