Physical Origin of Differences among various Measures of Solar Meridional Circulation

TL;DR

This paper explains the systematic differences between various measures of solar meridional circulation by analyzing the effects of surface turbulent magnetic diffusion, showing how magnetic diffusion influences feature-tracking speeds relative to plasma speeds.

Contribution

It provides a physical explanation for the differences in solar meridional flow measurements by linking magnetic diffusion effects to observed discrepancies.

Findings

Feature-tracking speeds are lower than plasma speeds at low/mid-latitudes.

Magnetic diffusion opposes poleward plasma flow at low latitudes.

Differences in speeds are consistent with flux-transport dynamo model predictions.

Abstract

We show that systematic differences between surface Doppler and magnetic element tracking measures of solar meridional flow can be explained by the effects of surface turbulent magnetic diffusion. Feature-tracking speeds are lower than plasma speeds in low and mid-latitudes, because magnetic diffusion opposes poleward plasma flow in low-latitudes whereas it adds to plasma flow at high latitudes. Flux transport dynamo models must input plasma flow; the model-outputs yield estimates of the surface magnetic feature tracking speed. We demonstrate that the differences between plasma speed and magnetic pattern speed in a flux-transport dynamo are consistent with the observed difference between these speeds.