Measuring deep solar meridional flow by a new strategy

Abstract:

Time-distance helioseismology studies solar interior flows by inverting acoustic travel-time differences. For the solar meridional flow, the global-scale travel-time measurements are dominated by the center-to-limb effect. To remove this effect, previous methods used the equatorial East-West travel time as a proxy, and measure the extra travel time in North-South direction along the central meridian for the interior meridional flow.

Here we present a new strategy to measure the deep meridional flow. Instead of measuring only East-West and North-South acoustic travel times, the new strategy measures travel times along a number of selected directions. The center-to-limb effect is presumably independent of the measuring direction, while the meridional flow contributes a cosine-angle component to the travel-time shifts. The center-to-limb effect and the travel-time differences caused by the interior meridional flow can thus be decoupled by solving a set of least-square equations. We finally show the isolated center-to-limb effect and inversion results of the solar meridional flow using four years SDO/HMI observation data.