Ring Analysis of Solar Subsurface Flows and Their Relation to Surface Magnetic Activity

Abstract

We measure the horizontal flows in the outer 2% of the Sun by analyzing 14 consecutive Carrington rotations of Global Oscillation Network Group (GONG) Doppler images and two of Michelson Doppler Imager (MDI) Dynamics Program data with the ring-diagram technique. The zonal and meridional flows show no variation with activity at low to medium activity levels (below 71 G). At active region locations, the zonal flow increases with increasing activity; active regions rotate faster than their quieter surroundings. The meridional flow at active region locations is more equatorward than on average at depths less than about 10 Mm; the flow converges toward the mean latitude of activity. At depths greater than about 10 Mm, some active region locations show poleward and others equatorward motions indicating strong outflows from active regions. The estimated vertical flow decreases with increasing activity levels except at active region locations at depths greater than about 10 Mm; active regions show downflows near the surface and upflows at depths greater than about 10 Mm. The velocity errors increase somewhat with increasing activity at flux levels below 71 G, but they increase rapidly up to about 2 times the median error at higher flux values. This increase occurs at all depths. The flows averaged over all longitudes show the patterns expected from solar cycle variations. The quiet and the intermediate activity subsets show the same flow pattern, while the active region subset shows a mixture of solar cycle flow pattern and local flow behavior.