Time-distance Measurements of Meridional Circulation Deep in
the Convection Zone


P. M. Giles
T. L. Duvall, Jr.
P. H. Scherrer

Department of Applied Physics, Stanford University
Laboratory for Astronomy and Solar Physics, NASA/GSFC
W.W. Hansen Experimental Physics Laboratory, Stanford University

pgiles@solar.stanford.edu

Explaining the solar cycle is one of the central goals of solar physics.  Some
of the most successful models of the cycle fall under the broad category of
Babcock-Leighton dynamo theories.  Babcock and Leighton developed this model
in the 1960s, making use of the most recent observations of the Sun's magnetic
field and surface motions.  The model reproduces the large-scale properties of
the cycle by invoking both differential rotation and supergranular diffusion
of magnetic elements.  Although the original work predates the birth of
helioseismology, it still underlies much of our current understanding of the
solar cycle.

The development of helioseismology has, however, necessitated some evolution
of the theory.  For example, dynamo theorists now must match their models to
the observed differential rotation profile in the solar interior.  Prodded by
more sophisticated surface measurements, several groups have also proposed
models including a meridional circulation.  Until recently, theorists were
free to speculate on the characteristics of this flow below the surface.  In
the past few years, however, several helioseismic techniques have been used to
successfully measure the meridional circulation in the solar interior.

In this paper, the authors present their latest measurements of the meridional
flow using the time-distance technique on MDI data.  These measurements now
reach far enough into the convection zone that they might be a useful
constraint on solar dynamo theories.