Observations of Time Variation in the Sun's Rotation. 
Observations of solar p-mode frequency splittings obtained at Big Bear Solar Observatory in 1986 and during 1988-90 reveal small ( approximately 1 percent) changes in the sun's subsurface angular velocity with solar cycle.  An asymptotic inversion of the splitting data yields the latitude dependence of the rotation rate and shows that the largest changes in the angular velocity, approximately 4 nanohertz, occurred between 1986 and the later years, at high ( approximately 60 degrees ) solar latitudes.  Earlier helioseismic observations suggest that solar cycle changes in the ratio of magnetic to turbulent pressure in the solar convection zone are large enough to account for the magnitude of the observed angular velocity variations but a detailed model of the phenomenon does not exist.