Abstract
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 B 0 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 B 0 angle are absent from the corrected flows.
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Acknowledgements
We thank the referee for helpful comments and catching a mistake. The data used here are courtesy of NASA/SDO and the HMI Science Team. We thank the HMI team members for their hard work. This work also utilizes GONG data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under a cooperative agreement with the National Science Foundation. This work was supported by NASA grant NNX11AQ57G and NSF/SHINE Award No. 1062054 to the National Solar Observatory. R.K. was partially supported by NASA grant NNX10AQ69G to Alysha Reinard.
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I. González Hernández is deceased.
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Komm, R., González Hernández, I., Howe, R. et al. Subsurface Zonal and Meridional Flow Derived from GONG and SDO/HMI: A Comparison of Systematics. Sol Phys 290, 1081–1104 (2015). https://doi.org/10.1007/s11207-015-0663-6
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DOI: https://doi.org/10.1007/s11207-015-0663-6