Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure

TL;DR

This study investigates how the structure of sunspots and penumbrae in MHD simulations depends on boundary conditions and grid resolution, finding certain features are robust while others vary with simulation parameters.

Contribution

It systematically analyzes the effects of boundary conditions and grid resolution on sunspot simulation results, highlighting the robustness of key features.

Findings

Inverse polarity flux is robust against resolution changes.

Overturning convective motions remain consistent across conditions.

Evershed flow channels are strongly magnetized at photospheric levels.

Abstract

MHD simulations of sunspots have successfully reproduced many aspects of sunspot fine structure as consequence of magneto convection in inclined magnetic field. We study how global sunspot properties and penumbral fine structure depend on the magnetic top boundary condition as well as on grid spacing. The overall radial extent of the penumbra is subject to the magnetic top boundary condition. All other aspects of sunspot structure and penumbral fine structure are resolved at an acceptable level starting from a grid resolution of 48 [24] km (horizontal [vertical]). We find that the amount of inverse polarity flux and the overall amount of overturning convective motions in the penumbra are robust with regard to both, resolution and boundary conditions. At photospheric levels Evershed flow channels are strongly magnetized. We discuss in detail the relation between velocity and magnetic field structure in the photosphere and point out observational consequences.