Surface flux transport simulations: Effect of inflows toward active regions and random velocities on the evolution of the Sun's large-scale magnetic field

TL;DR

This study uses surface flux transport simulations to analyze how inflows toward active regions and turbulent flows influence the evolution of the Sun's large-scale magnetic field, confirming diffusion as a valid model and revealing complex effects of inflows on magnetic flux and dipole moments.

Contribution

It introduces a simple flux transport model incorporating inflows and compares diffusion and random walk dispersal, highlighting inflows' dual role in flux cancellation and magnetic field configuration.

Findings

Diffusion effectively models large-scale flux dispersal.

Inflows increase flux cancellation and influence polarity separation.

Small initial tilt angles can lead to increased dipole moments due to inflows.

Abstract

Aims: We aim to determine the effect of converging flows on the evolution of a bipolar magnetic region (BMR), and to investigate the role of these inflows in the generation of poloidal flux. We also discuss whether the flux dispersal due to turbulent flows can be described as a diffusion process. Methods: We developed a simple surface flux transport model based on point-like magnetic concentrations. We tracked the tilt angle, the magnetic flux and the axial dipole moment of a BMR in simulations with and without inflows and compared the results. To test the diffusion approximation, simulations of random walk dispersal of magnetic features were compared against the predictions of the diffusion treatment. Results: We confirm the validity of the diffusion approximation to describe flux dispersal on large scales. We find that the inflows enhance flux cancellation, but at the same time affect the latitudinal separation of the polarities of the bipolar region. In most cases the latitudinal separation is limited by the inflows, resulting in a reduction of the axial dipole moment of the BMR. However, when the initial tilt angle of the BMR is small, the inflows produce an increase in latitudinal separation that leads to an increase in the axial dipole moment in spite of the enhanced flux destruction. This can give rise to a tilt of the BMR even when the BMR was originally aligned parallel to the equator.