Subsurface Zonal and Meridional Flow During Cycles 23 and 24

Abstract

We study the solar-cycle variation of subsurface flows from the surface to a depth of 16 Mm. We have used ring-diagram analysis to analyze 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 combined the zonal and meridional flows from the three data sources and scaled the flows derived from MDI and GONG to match those from HMI observations. In this way, we derived their temporal variation in a consistent manner for Solar Cycles 23 and 24. We have corrected the measured flows for systematic effects that vary with disk positions. Using time-depth slices of the corrected subsurface flows, we derived the amplitudes and times of the extrema of the fast and slow zonal and meridional flows during Cycles 23 and 24 at every depth and latitude. We find an average difference between maximum and minimum amplitudes of $8.6 \pm0.4~\mbox{m}\,\mbox{s}^{-1}$ for the zonal flows and $7.9 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the meridional flows associated with Cycle 24 averaged over a depth range from 2 to 12 Mm. The corresponding values derived from GONG data alone are $10.5 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the zonal and $10.8 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the meridional flow. For Cycle 24, the flow patterns are precursors of the magnetic activity. The timing difference between the occurrence of the flow pattern and the magnetic one increases almost linearly with increasing latitude. For example, the fast zonal and meridional flow appear $2.1 \pm 0.6$ years and $2.5\pm 0.6$ years, respectively, before the magnetic pattern at $30^{\circ}$ latitude in the northern hemisphere, while in the southern hemisphere, the differences are $3.2 \pm 1.2$ years and $2.6 \pm 0.6$ years. The flow patterns of Cycle 25 are present and have reached $30^{\circ}$ latitude. The amplitude differences of Cycle 25 are about 22% smaller than those of Cycle 24, but are comparable to those of Cycle 23. Moreover, polynomial fits of meridional flows suggest that equatorward meridional flows (counter-cells) might exist at about $80^{\circ}$ latitude except during the declining phase of the solar cycle.