Magnetohydrodynamics of the Weakly Ionized Solar Photosphere

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 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 −1 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.