Measuring Acoustic Travel Times in Higher-Latitude Regions of the Sun using Hinode and SDO Data

The interior structure and dynamics of the Sun can be probed by measuring and inverting travel times of acoustic waves, widely known as time-distance helioseismology. Recent high-resolution observations of solar oscillations with Hinode/SOT and SDO/HMI provide us with an opportunity to investigate the flow dynamics in higher-latitude regions of the Sun. Of particular interest is the meridional circulation flow, which is crucial for understanding the solar dynamo mechanism and predicting the solar activity cycles. We investigate systematic uncertainties of the travel times due to the center-to-limb variations, which may significantly affect the helioseismic inferences of the meridional flows. We present the results of analysis of the cross-correlations in the Doppler velocity, line core and intensity observations, and the corresponding travel-time fits for various positions on the solar disk. We discuss the origin of the center-to-limb variations, including the foreshortening effect, the difference in the line formation height, and other effects. For better understanding of the relative role of these effects we use 3D numerical simulations of solar oscillations in a realistic model of the Sun.