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 thermal structure. Like differential rotation (zonal flow), meridional flow may play both an active role in the dynamo, by transporting magnetic flux, and a passive role, by serving as an indicator of magnetic changes. Prior helioseismic analysis of meridional flow is consistent with a conveyor-belt pattern of flow in each hemisphere, having a poleward velocity of approximately 15 m/s in the upper convection zone and an equatorward flow of several m/s amplitude near the base of the convection zone. However, the flow pattern in the lower half of the convection zone is not observationally well constrained by analysis to date. Observations also suggest that additional cells may be present. 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 frequency splitting of oscillation multiplets of order n and degree l, due to differential rotation. Mode coupling also produces a signature in cross spectra of distinct spherical harmonic time series. In this presentation I discuss the problem of using of 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 will be presented.

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