Is the Subsurface Meridional Flow Zero at the Equator?

Abstract

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 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. The meridional flow at the equator is small but nonzero and is mainly negative (southward) during Solar Cycle 23 with an average of $-1.1 \pm 0.2~\text{m}\,\text{s}^{-1}$ at depths shallower than 7 Mm and positive (northward) during Solar Cycle 24 with an average of $+1.3 \pm 0.1~\text{m}\,\text{s}^{-1}$ over the same depth range derived from supersynoptic maps of combined HMI and GONG data (scaled to match HMI flow amplitudes). The divergence in supersynoptic maps is positive at all times and clearly varies with the solar cycle with large values during cycle maxima and small values during minima. On time scales of synoptic maps, we found that at depths shallower than 10 Mm the cross-equatorial flow is, on average, toward the hemisphere with the larger amount of flux. The meridional flow at the equator has broad distributions with widths that are at least five times larger than the mean values. The distributions of Solar Cycles 23 and 24 overlap but are distinguishable. For a high-activity subset, the cross-equatorial flow is predominantly toward locations with high activity and the divergence is greater than average. The nonzero cross-equatorial flow is in this case a consequence of the inflows present near active regions and the imbalance of activity between the hemispheres. For a quiet-region subset, the cross-equatorial flow is, on average, in the same direction as the average flow over a solar cycle with a similar broad distribution, while the quiet-region divergence is smaller than the grand average.