H.M. Antia (Tata Institute of Fundamental Research, Mumbai, India)

We present results from time-distance helioseismic measurements of meridional circulation in the solar convection zone using 4 years of Doppler velocity observations by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We show, by taking into account the limitations of decreasing signal-to-noise in the travel time measurements of acoustic waves that sample deeper layers and through an in-built mass conservation constraint in the inversion scheme, that the return flow that closes the meridional circulation is possibly below a depth of about 0.77 R_sun. We discuss the significance of this result in relation to other helioseismic inferences published recently and possible reasons for the differences in the results. We also discuss the implications of our results for the dynamics of solar interior and popular solar dynamo models.

After reviewing the well established observational results on solar meridional flows in the near surface layers, I discuss recent time-distance helioseismic measurements on the deep structure of the circulation. I also discuss the limitations of such analyses due to noise and large systematics of unknown origins. Finally, I discuss implications for theoretical/numerical models of meridional circulation as well as for those which use it in modeling the solar dynamo.

H.M. Antia

We present results on travel time measurements for different frequencies through Gaussian frequency filtering of time-distance cross-correlation signals. We show that the center-to-limb systematics inferred through WE travel time measurements have a dominant frequency dependence, peaking at about 4 mHz. The subtraction of such travel times from the NS travel times leads to a strong frequency dependence in the resulting meridional flow signals too thus yielding flow structures dependent on frequency. In particular, we find multiple-cell structure for frequencies above 3.7 mHz.

H.M. Antia, S.M. Hanasoge, K. Mondal

Inversions for solar meridional flows from frequency dependent travel times are presented. Born approximation kernels for meridional flows and tests on model flows are also presented.