Average Subsurface Flows of Active and Quiet Regions with Large-Tile Ring-Diagram Analysis

Abstract

We study large-scale subsurface flows associated with locations of high and low magnetic activity in the near-surface shear layer (NSSL). The flows were derived with ring-diagram analysis applied to Helioseismic and Magnetic Imager (HMI) Dopplergrams using 30^∘ tiles. The zonal and meridional flows of the high- and low-activity subsets show the expected variation with the solar cycle with the low-activity subset showing flow precursors of cycle activity. We then focus on temporal averages of these flows. The average zonal flow of the high-activity subset is faster than that of the low-activity one at 0^∘ and 15^∘, while they are about the same size at 30^∘ latitude. The amplitudes of the average meridional flow of the high-activity subset is smaller than those of the low-activity one at 30^∘ and comparable at 15^∘ latitude. The differences between the high- and low-activity subsets indicate the contribution of active regions to the flow pattern. The extra zonal flow of active regions increases with increasing depth, while the extra meridional flow converges near locations of activity. The converging flow is strongest at depths less than about 5 Mm, decreases with increasing depth, and is rather constant at depths greater than about 10 Mm. We then calculate the radial gradients of both flow components and find that those of the high-activity subset are generally more negative than those of the low-activity one. The extra radial gradients of active regions are about the same for both flow components, while the radial gradient of the zonal flow is about one order of magnitude larger than that of the meridional flow. Finally, we determine the radial extent of the NSSL using the radial gradient of the zonal flow and find that the NSSL extends to about 29 Mm at high-activity locations compared to 35 Mm at low-activity ones, or to about 83% of its depth at low-activity locations.