Subsurface Horizontal Flows During Solar Cycles 24 and 25 with Large-Tile Ring-Diagram Analysis

We study the large-scale subsurface flows to a depth of about 32 Mm covering the near-surface shear layer (NSSL). The flows were derived with a ring-diagram analysis applied to Helioseismic and Magnetic Imager (HMI) Dopplergrams using tiles with 30 ∘ diameter instead of the commonly used 15 ∘ ones. This allows us to determine flows at greater depths in exchange for coarser spatial resolution. We confirm that the average zonal flow increases with increasing depth and reaches a plateau in the NSSL. There is a hint of a local maximum or saddle point much closer to the surface at about 8 Mm. The average meridional flow is poleward at all depths in both hemispheres. The average amplitude is 14.3 ± 0.2 m s −1 at 30 ∘ and 12.4 ± 0.2 m s −1 at 15 ∘ latitude at depths of 20 Mm and shallower, while amplitudes at these latitudes decrease at greater depths. The solar-cycle variation of the zonal and meridional flow are clearly noticeable from the surface throughout the NSSL. The dominant features of the zonal flow are bands of faster-than-average flow associated with Solar Cycles 24 and 25. The onset of the fast bands happens almost simultaneously at all depths. For Cycle 25, the fast bands appear in the southern hemisphere about one year before those in the northern one and both fast bands appear several years before magnetic activity appeared at the surface in either hemisphere. The meridional flow shows a similar pattern after subtracting the temporal mean at each latitude. The bands of converging residual meridional flow move from mid- to low latitudes during a solar cycle. These bands appear at low latitudes almost at the same time at all depths, similar to the fast bands of the zonal flow. However, at 45 ∘ latitude they appear first in layers near 32 Mm and about two years later at the solar surface, as if the pattern were rising through the outer layers.