Subsurface Zonal and Meridional Flow Derived from GONG and SDO/HMI: A Comparison of Systematics

We study the subsurface flows in the near-surface layers of the solar convection zone from the surface to a depth of 16 Mm derived from Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) Dopplergrams using a ring-diagram analysis. We characterize the systematic east–west and north–south variations present in the zonal and meridional flows and compare flows derived from GONG and HMI data before and after the correction. The average east–west variation with depth of one flow component resembles the average north–south variation with depth of the other component. The east–west variation of the zonal flow together with the north–south variation of the meridional flow can be modeled as a systematic radial velocity. This indicates a solar center-to-limb variation as the underlying cause. The north–south variation of the zonal flow and the east–west variation of the meridional flow require two separate functions. The east–west variation of the meridional flow consists mainly of an annual variation with the B0 angle, while the north–south trend of the zonal flow consists of a constant non-zero component in addition to an annual variation. This indicates a geometric projection artifact. After compensating for these systematic effects, the meridional and zonal flows derived from HMI data agree well with those derived from GONG data. An offset remains between the zonal flow derived from GONG and HMI data. The equatorward meridional flows at high latitude that appear episodically depending on the B0 angle are absent from the corrected flows.