Meridional circulation in the solar convection zone: time-distance helioseismic inferences from four years of HMI/SDO observations

TL;DR

This study uses four years of helioseismic data to infer the meridional circulation in the solar convection zone, revealing a possible return flow beneath 0.77 solar radii and discussing implications for solar interior dynamics.

Contribution

It introduces a new inversion method with a mass conservation constraint to analyze helioseismic travel times for meridional circulation in the solar interior.

Findings

Return flow possibly beneath 0.77 R_sun

Highlights measurement challenges and systematics

Implications for solar dynamo models

Abstract

We present and discuss 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). Using an in-built mass conservation constraint in terms of the stream function we invert helioseismic travel times to infer meridional circulation in the solar convection zone. We find that the return flow that closes the meridional circulation is possibly beneath the depth of $0.77 R_{\odot}$. We discuss the significance of this result in relation to other helioseismic inferences published recently and possible reasons for the differences in the results. Our results show clearly the pitfalls involved in the measurements of material flows in the deep solar interior given the current limits on signal-to-noise and our limited understanding of systematics in the data. We also discuss the implications of our results for the dynamics of solar interior and popular solar dynamo models.