Emergence of a flux tube through a partially ionised solar atmosphere

TL;DR

This study extends 3D simulations of magnetic flux emergence through the partially ionised solar atmosphere, demonstrating the importance of neutrals in accurately modeling current removal, temperature profiles, and stability.

Contribution

It introduces 3D modeling of flux emergence including neutrals, showing their critical role in realistic temperature and stability outcomes.

Findings

Cowling resistivity effectively removes perpendicular currents in 3D.

Including neutrals prevents unphysical temperature drops and Rayleigh-Taylor instability.

A simplified neutral layer model can replicate full simulations when spectral emission is not needed.

Abstract

For a magnetic flux tube, or indeed any flux, to emerge into the Solar corona from the convection zone it must pass through the partially ionised layers of the lower atmosphere: the photosphere and the chromosphere. In such regions the ion-neutral collisions lead to an increased resistivity for currents flowing across magnetic field lines. This Cowling resistivity can exceed the Spitzer resistivity by orders of magnitude and in 2.5D simulations has been shown to be sufficient to remove all cross field current from emerging flux. Here we extend this modelling into 3D. Once again it is found that the Cowling resistivity removes perpendicular current. However the presence of 3D structure prevents the simple comparison possible in 2.5D simulations. With a fully ionised atmosphere the flux emergence leads to an unphysically low temperature region in the overlying corona, lifting of chromospheric material and the subsequent onset of the Rayleigh-Taylor instability. Including neutrals removes the low temperature region, lifts less chromospheric matter and shows no signs of the Rayleigh-Taylor instability. Simulations of flux emergence therefore should include such a neutral layer in order to obtain the correct perpendicular current, remove the Rayleigh-Taylor instability and get the correct temperature profile. In situations when the temperature is not important, i.e. when no simulated spectral emission is required, a simple model for the neutral layer is demonstrated to adequately reproduce the results of fully consistent simulations.