Thomas L. Duvall Jr., Laurent Gizon

In this study, we apply time-distance helioseismology to the medium-degree Dopplergrams observed by SOHO/MDI and SDO/HMI, in order to measure meridional circulation in the deep convection zone. The results are averaged over the same observation period (May 2010 to Apr 2011) of the two data set and then compared with each other. After the removal of the known center-to-limb variation and a roll-angle uncertainty in MDI data by registering with HMI data, the travel-time results obtained from MDI and HMI data are consistent. Even the fluctuations in the travel-time differences match closely. Furthermore, we analyse 4-year HMI data and exclude the active regions from our analysis. The travel-time measurements of the meridional flow using the HMI data show a time variation in the convection zone similar to that using MDI data measured by Liang & Chou (2015).