Magnetic feature tracking, what determines the speed?

TL;DR

This paper investigates the factors influencing the speed of magnetic features on the solar surface, examining how plasma velocity and diffusive processes affect magnetic element movement, and explores alternative explanations for observed discrepancies.

Contribution

It analyzes the impact of radial velocity gradients on magnetic feature tracking speed and proposes alternative mechanisms beyond turbulent magnetic pumping.

Findings

Magnetic tracers can move faster than plasma if the radial velocity gradient is negative.

Turbulent magnetic pumping must be unrealistically strong to match observations.

The sign of the radial velocity gradient critically influences magnetic feature motion.

Abstract

Recent observations revealed that small magnetic elements abundant at the solar surface move poleward with a velocity which seems to be lower than the plasma velocity $U_{\theta}$. Guerrero et al. (2011) explained this discrepancy as a consequence of diffusive spreading of the magnetic elements due to a positive radial gradient of $|U_{\theta}|$. As the gradient's sign (inferred by local helioseismology) is still unclear, cases with a negative gradient are studied in this paper. Under this condition, the velocity of the magnetic tracers turns out to be larger than the plasma velocity, in disagreement with the observations. Alternative mechanisms for explaining them independently are proposed. For the turbulent magnetic pumping it is shown that it has to be unrealistically strong to reconcile the model with the observations.