Comparison of acoustic travel-time measurements of solar meridional circulation from SDO/HMI and SOHO/MDI

Context. Time-distance helioseismology is one of the primary tools for studying the solar meridional circulation, especially in the lower convection zone. However, travel-time measurements of the subsurface meridional flow suffer from a variety of systematic errors, such as a center-to-limb variation and an off set due to the position angle (P-angle) uncertainty of solar images. It has been suggested that the center-to-limb variation can be removed by subtracting east-west from south-north travel-time measurements. This ad hoc method for the removal of the center-to-limb effect has been adopted widely but not tested for travel distances corresponding to the lower convection zone. Aims. We explore the effects of two major sources of the systematic errors, the P-angle error arising from the instrumental misalignment and the center-to-limb variation, on the acoustic travel-time measurements in the south-north direction. Methods. We apply the time-distance technique to contemporaneous medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the travel-time difference caused by meridional circulation throughout the solar convection zone. The P-angle off set in MDI images is measured by cross-correlating MDI and HMI images. The travel-time measurements in the south-north and east-west directions are averaged over the same observation period (May 2010 to Apr. 2011) for the two data sets and then compared to examine the consistency of MDI and HMI travel times after applying the above-mentioned corrections. Results. The off sets in the south-north travel-time difference from MDI data induced by the P-angle error gradually diminish with increasing travel distance. However, these off sets become noisy for travel distances corresponding to waves that reach the base of the convection zone. This suggests that a careful treatment of the P-angle problem is required when studying a deep meridional flow. After correcting the P-angle and the removal of the center-to-limb effect, the travel-time measurements from MDI and HMI are consistent within the error bars for meridional circulation covering the entire convection zone. The fluctuations observed in both data sets are highly correlated and thus indicate their solar origin rather than an instrumental origin. Although our results demonstrate that the ad hoc correction is capable of reducing the wide discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude the possibility that there exist other systematic effects acting on the two data sets in the same way.