Measurement and Interpretation of Travel-Time Shifts in the context of Time-Distance Helioseismic Detection of Meridional Flows in the Solar Convection Zone

The role of meridional flow in maintaining the solar dynamo and differential rotation in the solar convection zone is not well understood and is currently under scrutiny. The traditional flux-transport dynamo models have posited the well known single-cell meridional flow with poleward flow at the photosphere and equatorward flow near the base of the convection zone. However, recent investigations seem to be revealing a different picture of meridional flow which is double celled in the radial direction with poleward flow at the photosphere and equatorward flow at a much shallower level in the convection zone. In this work time-distance helioseismology is used to probe the solar convection zone to accurately determine the structure of meridional circulation.

Helioseismology uses the photospherically visible aspect of (acoustic, surface-gravity) waves, that propagate and interfere throughout the Sun to form standing oscillation modes, as probes to make inferences about the structure and flows on the solar surface and interior. Time-distance helioseismology is based on measuring the travel-times of wave-packets moving between distinct points on the solar surface. Travel-time shifts obtained by calculating the difference in the travel-times of counter-propagating waves between the same points on the solar surface yield information about flows throughout the solar convection zone. In this work time-distance techniques are applied on artificial and solar Doppler velocity images to detect travel-time shifts due to meridional flow. Modifications are suggested to enhance the signal-to-noise ratio of travel-time shift measurements. The artificial data is constructed by embedding various meridional flow models in 3D acoustic simulators, which is then used to discuss the interpretation of travel-time shifts, so that in the future an inversion procedure may be designed to calculate meridional flow velocities with greater accuracy. The solar data is obtained from the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO) spacecraft and is used to measure travel-time shifts due to meridional flow. The issue of a systematic error dubbed as the ‘center-to-limb effect’ that contaminates the travel-time shift measurements of solar meridional flow is also addressed in this work.