Helioseismic Measurement of Large-Scale Solar Flows

Large-scale flows in the deep convection zone, differential rotation and meridional circulation in particular, are relatively accessible to seismic probing and provide observational constraints on convection-zone dynamics and structure. Flows and other aspherical perturbations dynamically couple the Sun's normal oscillation modes. Mode coupling produces signatures in power spectra of time series of coefficients in the spherical harmonic decomposition of the photospheric velocity field, the outstanding signature being the consequence of azimuthal-order (m) dependence of the mode frequencies due to differential rotation. Mode coupling also produces a signature in cross spectra of distinct spherical-harmonic time series. In this paper I discuss the problem of using helioseismic cross-spectra to map large-scale flow in the Sun and prospects for measuring deep meridional flow. Some preliminary estimates of meridional and zonal flow from MDI and GONG spherical-harmonic time series are presented.