  A helioseismic statistical waveform analysis of subsurface flow was performed
on two 720-day time series of SOHO/MDI Medium-l spherical-harmonic
coefficients. The time series coincide with epochs of high and low solar
activity. Time-dependent coupling-strength coefficients b(s,t;n,l) of modes of
the same radial order n and degree l, but different azimuthal order m, were
inferred from the waveform analysis. These coefficients are sensitive to flows
and general aspherical structure. For odd values of s << l, the coefficient
b(s,t;n,l) measures an average over depth of the amplitude of one
spherical-harmonic (s,t) component of the toroidal flow velocity field. The
depth-dependent weighting function defining the average velocity is the
fractional kinetic energy density in radius of modes of the (n,l) multiplet. A
mean-square (n,l)-dependent flow velocity was inferred from the b-coefficients
for s in the range 5 through 35 for each n and l in the respective ranges 1
through 5 and 120 through 149 for the epochs of high and low activity. A
further averaging, over l, yielded a root mean square flow velocity as a
function of n for each epoch, which average increases from about 20 m/s at n=1
to 35 m/s at n=5. The inferred velocities are consistent with (though perhaps
do not demand) a cellular pattern of flow extending over the vertical range of
mode sensitivity, estimated to be about four percent of the solar radius below
the photosphere.
