Solar-Cycle Variation of the Subsurface Flows of Active- and Quiet-Region Subsets

Abstract

We study the solar-cycle variation of subsurface flows for both active and quiet solar regions. We derive flows from the surface to a depth of 16 Mm using ring-diagram analysis applied to Dopplergrams obtained with the Michelson Doppler Imager (MDI) Dynamics Program, the Global Oscillation Network Group (GONG), and the Helioseismic and Magnetic Imager (HMI) instrument. We derive the temporal variation of the zonal and meridional flows in a consistent manner for Solar Cycles 23 and 24 combining the flows from the three data sources scaled to match HMI-derived flows. The subsurface flows associated with active and quiet regions show the same variation with the solar cycle with alternating bands of faster- and slower-than-average zonal and meridional flows moving from mid-latitudes toward the equator during the course of a cycle. We derive the differences between the amplitudes of the extrema of the fast and the slow flows. For Cycle 24, the average difference between the fast- and slow-flow amplitude is $9.5 \pm 0.5~\text{m}\,\text{s}^{-1}$ for the zonal flows and $7.0 \pm 0.4~\text{m}\,\text{s}^{-1}$ for the meridional flows of the quiet-region subset averaged over 2 to 12 Mm within $\pm 30^{\circ}$ latitude. For the active-region subset, the average difference is $10.4 \pm 0.9~\text{m}\,\text{s}^{-1}$ for the zonal flows and $9.3 \pm 0.7~\text{m}\,\text{s}^{-1}$ for the meridional flows. We subtract the flows of the quiet-region subset from those of the active-region one to determine the contribution of active regions to the long-term flow pattern. The resulting meridional flow associated with active regions has a maximum amplitude near 3.1 Mm and its amplitude decreases with depth. This implies that the converging flows attributed to active regions are a shallow-layer phenomenon.