Abstract
We describe and apply a new helioseismic method for measuring solar subsurface axisymmetric meridional and zonal flow. The method is based on a theoretical model of the response of global-oscillation eigenfunctions to the flow velocity and uses cross spectra of the time-varying coefficients in the spherical-harmonic expansion of the photospheric Doppler-velocity field. Eigenfunction changes modify the leakage matrix, which describes the sensitivity of the spherical-harmonic coefficients to the global-oscillation modes. The form of the leakage matrix in turn affects the theoretically expected spherical-harmonic cross spectra. Estimates of internal meridional and zonal flow were obtained by fitting the theoretical flow-dependent cross spectra to spherical-harmonic cross spectra computed from approximately 500 days of full-disk Dopplergrams from the Helioseismic and Magnetic Imager (HMI) on the SDO spacecraft. The zonal-flow measurements, parameterized in the form of “a” coefficients, substantially agree with measurements obtained from conventional global-mode-frequency analysis. The meridional-flow estimates, in the form of depth-weighted averages of the flow velocity, are similar to estimates obtained from earlier analyses, for oscillation modes that penetrate the outermost one-third of the convection zone. For more deeply penetrating modes, the inferred flow velocity increases significantly with penetration depth, indicating the need for either a modification of the simple conveyor-belt picture of meridional flow or improvement in the cross-spectral model.
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Acknowledgements
We thank Doug Braun for useful discussions and Ashley Crouch for useful discussions and calculations. HMI data are courtesy of NASA/SDO and the HMI science team. This research was supported by NASA contracts NAS5-03114 and NNH09CF93C to NWRA/CoRA. HMI is supported by NASA contract NAS5-02139 to Stanford University.
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Solar Dynamics and Magnetism from the Interior to the Atmosphere
Guest Editors: R. Komm, A. Kosovichev, D. Longcope, and N. Mansour
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Woodard, M., Schou, J., Birch, A.C. et al. Global-Oscillation Eigenfunction Measurements of Solar Meridional Flow. Sol Phys 287, 129–147 (2013). https://doi.org/10.1007/s11207-012-0075-9
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DOI: https://doi.org/10.1007/s11207-012-0075-9