Detection of Equatorward Meridional Flow and Evidence of Double-Cell Meridional Circulation inside the Sun
Determining the internal meridional flow is one important task in solar physics and yet one difficult challenge due to the low speed of the flow, which requires a careful analysis of potential systematic errors. In particular, recent studies have revealed a previously unreported systematic center-to-limb effect in the acoustic travel times measured by the time-distance technique (Zhao et al. 2012). The systematic travel-time variation depends on the angular distance from the solar disk center, and exhibits different magnitude, sometimes even opposite signs, when measured using different HMI observables and using different measurement distances. It is not quite clear what causes this systematic effect, and a recent study suggested that it might be partially due to the interactions of acoustic waves with the vertical flows in solar granules (Baldner & Schou 2012). Zhao et al. (2012) suggested that this systematic effect should be removed before inverting the measured acoustic travel times for interior meridional flow, and proposed to use the travel-time shifts measured in the east-west direction along the equator as proxies of the systematic effect. Their inversion results, after removal of this effect following their suggested method, showed a reduction of ~ 10 m s" in the inverted meridional flow speed (Zhao et al. 2012). We believe that this systematic effect was responsible for many past helioseismology study failures to reliably detect the equatorward meridional flow, and that the removal of this effect will play an important role in studying the meridional flow in the solar interior.