VARIATIONS IN THE AXISYMMETRIC TRANSPORT OF MAGNETIC ELEMENTS ON THE SUN: 1996-2010

We measure the axisymmetric transport of magnetic flux on the Sun by cross-correlating narrow strips of data from line-of-sight magnetograms obtained at a 96 minute cadence by the MDI instrument on the ESA/NASA SOHO spacecraft and then averaging the flow measurements over each synodic rotation of the Sun. Our measurements indicate that the axisymmetric flows vary systematically over the solar cycle. The differential rotation is weaker at maximum than at minimum. The meridional flow is faster at minimum and slower at maximum. The meridional flow speed on the approach to the Cycle 23/24 minimum was substantially faster than it was at the Cycle 22/23 minimum. The average latitudinal profile is largely a simple sinusoid that extends to the poles and peaks at about 35 degrees latitude. As the cycle progresses, a pattern of inflows toward the sunspot zones develops and moves equatorward in step with the sunspot zones. These inflows are accompanied by the torsional oscillations. This association is consistent with the effects of the Coriolis force acting on the inflows. The equatorward motions associated with these inflows are identified as the source of the decrease in net poleward flow at cycle maxima. We also find polar countercells (equatorward flow at high latitudes) in the south from 1996 to 2000 and in the north from 2002 to 2010. We show that these measurements of the flows are not affected by the nonaxisymmetric diffusive motions produced by supergranulation.