Plasma flow vs. magnetic feature-tracking speeds in the Sun

TL;DR

This study compares plasma flow speeds with magnetic feature-tracking speeds in the Sun using 1D and 2D models, revealing that in 1D they match the flow speed, but differ in 2D depending on diffusivity and velocity gradients.

Contribution

It demonstrates the differences in magnetic feature-tracking speeds between 1D and 2D models and how they depend on diffusivity and velocity gradients.

Findings

In 1D models, MFT speed equals the meridional flow speed.

In 2D models, MFT speed differs from flow speed based on diffusivity and velocity gradients.

Results confirmed with two different simulation codes.

Abstract

We simulate the magnetic feature tracking (MFT) speed using advective-diffusive transport models in both one and two dimensions. By depositing magnetic bipolar regions at different latitudes at the Sun's surface and following their evolution for a prescribed meridional circulation and magnetic diffusivity profiles, we derive the MFT speed as a function of latitude. We find that in a one dimensional surface-transport model the simulated MFT speed at the surface is always the same as the meridional flow-speed used as input to the model, but is different in a two-dimensional transport model in the meridional (r,theta) plane. The difference depends on the value of the magnetic diffusivity and on the radial gradient of the latitudinal velocity. We have confirmed our results with two different codes in spherical and Cartesian coordinates.