Magnetohydrodynamics of the Weakly Ionized Solar Photosphere

TL;DR

This study extends a solar photosphere simulation code to include ambipolar diffusion and Hall currents, revealing their increasing significance at high resolutions and their impact on magnetic and thermodynamic features.

Contribution

The paper introduces a generalized Ohm's law implementation in the MURaM code for realistic photospheric simulations, highlighting the importance of neutral-ion effects at high resolutions.

Findings

Hall currents and ambipolar diffusion become significant at 5-20 km resolution.

Neutral-ion effects cause flow changes of about 100 m/s in sunspot light bridges.

These effects alter the thermodynamic structure of quiet-Sun magnetic features.

Abstract

We investigate the importance of ambipolar diffusion and Hall currents for high-resolution comprehensive ('realistic') photospheric simulations. To do so we extended the radiative magnetohydrodynamics code \emph{MURaM} to use the generalized Ohm's law under the assumption of local thermodynamic equilibrium. We present test cases comparing analytical solutions with numerical simulations for validation of the code. Furthermore, we carried out a number of numerical experiments to investigate the impact of these neutral-ion effects in the photosphere. We find that, at the spatial resolutions currently used (5-20 km per grid point), the Hall currents and ambipolar diffusion begin to become significant -- with flows of 100 m/s in sunspot light bridges, and changes of a few percent in the thermodynamic structure of quiet-Sun magnetic features. The magnitude of the effects is expected to increase rapidly as smaller-scale variations are resolved by the simulations.