# Magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion

## TL;DR

This paper presents the first simulations of ambipolar diffusion in neutron star cores, showing evolution towards stable twisted torus magnetic configurations under simplified assumptions.

## Contribution

It introduces the first spherical, axisymmetric simulations of ambipolar diffusion in neutron stars, analyzing equilibrium states and their stability.

## Key findings

- Magnetic fields evolve towards barotropic equilibria satisfying the Grad-Shafranov equation.
- Equilibria are twisted torus configurations with poloidal and toroidal components.
- Axial symmetry equilibria appear stable, but may be unstable to non-axisymmetric perturbations.

## Abstract

As another step towards understanding the long-term evolution of the magnetic field in neutron stars, we provide the first simulations of ambipolar diffusion in a spherical star. Restricting ourselves to axial symmetry, we consider a charged-particle fluid of protons and electrons carrying the magnetic flux through a motionless, uniform background of neutrons that exerts a collisional drag force on the former. We also ignore the possible impact of beta decays, proton superconductivity, and neutron superfluidity. All initial magnetic field configurations considered are found to evolve on the analytically expected time-scales towards "barotropic equilibria" satisfying the "Grad-Shafranov equation", in which the magnetic force is balanced by the degeneracy pressure gradient, so ambipolar diffusion is choked. These equilibria are so-called "twisted torus" configurations, which include poloidal and toroidal components, the latter restricted to the toroidal volumes in which the poloidal field lines close inside the star. In axial symmetry, they appear to be stable, although they are likely to undergo non-axially symmetric instabilities.