Surface flux evolution constraints for flux transport dynamos

TL;DR

This study compares flux transport dynamo models with surface flux transport models to better understand the Sun's magnetic field evolution, highlighting the importance of downward magnetic flux pumping and boundary conditions.

Contribution

It demonstrates how different boundary conditions and diffusivity profiles affect flux transport dynamo models' agreement with surface flux transport models, emphasizing the role of downward flux pumping.

Findings

Downward magnetic flux pumping significantly slows flux diffusion.

Vertical boundary conditions align FTD with SFT when pumping is strong.

Potential field boundary conditions do not match SFT results.

Abstract

The surface flux transport (SFT) model of solar magnetic fields involves empirically well-constrained velocity and magnetic fields. The basic evolution of the Sun's large-scale surface magnetic field is well described by this model. The azimuthally averaged evolution of the SFT model can be compared to the surface evolution of the flux transport dynamo (FTD), and the evolution of the SFT model can be used to constrain several near-surface properties of the FTD model. We compared the results of the FTD model with different upper boundary conditions and diffusivity profiles against the results of the SFT model. Among the ingredients of the FTD model, downward pumping of magnetic flux, related to a positive diffusivity gradient, has a significant effect in slowing down the diffusive radial transport of magnetic flux through the solar surface. Provided the pumping was strong enough to give rise to a downflow of a magnetic Reynolds number of 5 in the near-surface boundary layer, the FTD using a vertical boundary condition matches the SFT model based on the average velocities above the boundary layer. The FTD model with a potential field were unable to match the SFT results.