Frequency-dependent Measurements of the Sun’s Meridional Circulation

The Sun’s meridional circulation is a crucial component for understanding the Sun’s interior dynamics and dynamo. However, currently its measurements from different authors still differ, especially in the number of circulation cells: single, double, or multiple? One major challenge in determining the meridional flow is how to remove a systematic center-to-limb (CtoL) effect, which could be 5-10 times stronger than the meridional-flow-caused travel-time shifts in the deep regions. Recently, it was found by Chen & Zhao (2018) that the CtoL effect measured in time-distance helioseismology has a strong acoustic-frequency dependence, which reverses sign at a certain frequency; in contrast, the flow-caused travel-time shifts change little with frequency. The significant differences in the frequency dependence indicate a promising way to disentangle the CtoL effect and the flow-induced signals in the Fourier domain, and therefore give a potentially more robust solution of the meridional circulation. In this work, we develop a new technique for making frequency-dependent time-distance measurements, and show preliminary results of the meridional circulation derived using multiple frequency bands.