Variations in the axisymmetric transport of magnetic elements on the Sun: 1996-2010

TL;DR

This study analyzes the systematic variations in the Sun's axisymmetric magnetic flux transport over solar cycles 23 and 24, revealing cycle-dependent flow patterns, in-flows toward sunspot zones, and polar counter-cells, using magnetogram data.

Contribution

It provides detailed measurements of axisymmetric flows on the Sun and their variation with the solar cycle, including the identification of in-flows and polar counter-cells.

Findings

Differential rotation weaker at solar maximum

Meridional flow faster at minimum

In-flows toward sunspot zones develop and move equatorward

Abstract

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 $35\degr$ latitude. As the cycle progresses a pattern of in-flows toward the sunspot zones develops and moves equatorward in step with the sunspot zones. These in-flows are accompanied by the torsional oscillations. This association is consistent with the effects of the Coriolis force acting on the in-flows. The equatorward motions associated with these in-flows are identified as the source of the decrease in net poleward flow at cycle maxima. We also find polar counter-cells (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 non-axisymmetric diffusive motions produced by supergranulation.