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ISWS Colloquium

We study the subsurface meridional flow and its divergence from the surface to a depth of about 16 Mm at the equator and its variation with the solar cycle. The flows were derived with ring-diagram analysis applied to Michelson Doppler Imager (MDI) Dynamics Program, Global Oscillation Network Group (GONG), and Helioseismic and Magnetic Imager (HMI) Dopplergrams. We find that the meridional flow at the equator is small but non-zero and is mainly negative or southward during Solar Cycle 23 and positive or northward during Solar Cycle 24 derived from super-synoptic maps of combined GONG and HMI data. The average divergence in super-synoptic maps is positive at all times, indicating a poleward meridional flow, and clearly varies with the solar cycle with large values during solar maximum and small values during cycle minimum. The enhanced divergence during epochs of high activity is most likely due to the inflows near active regions in both hemispheres. Analyzing synoptic maps, we found that the meridional flow at the equator is correlated with the North-South asymmetry of the magnetic activity at neighboring latitudes at depths of 10 Mm and shallower. The non-zero meridional flow at the equator is thus most likely a consequence of the inflows present near active regions and the imbalance of activity between the hemispheres.

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