Helioseismic Detection of Deep Meridional Flow

Abstract

Steady meridional flow does not make any first-order perturbations to the frequencies of helioseismic normal modes. It does, however, Doppler shift the local wavenumber, thereby distorting the eigenfunctions. For high-degree modes, whose peaks in a power spectrum are blended into continuous ridges, the effect of the distortion is to shift the locations of those ridges. From this blended superposition of modes, one can isolate oppositely directed wave components with the same local horizontal wavenumber and measure a frequency difference which can be safely used to infer the subsurface background flow. But such a procedure fails for the components of the more deeply penetrating low-degree modes that are not blended into ridges. Instead, one must analyze the spatial distortions explicitly. With a simple toy model, we illustrate a method by which that might be accomplished by measuring the spatial variation of the oscillation phase. We estimate that by using this procedure, it might be possible to infer meridional flow deep in the solar convection zone.