A new code for the Hall-driven magnetic evolution of neutron stars

TL;DR

This paper introduces a novel numerical code using upwind finite differences to model the complex, transitionary magnetic evolution in neutron star crusts, capable of handling sharp current sheets and low diffusivity.

Contribution

A new finite difference code that accurately models the Hall-driven magnetic evolution in neutron star crusts, overcoming previous spectral method limitations.

Findings

Successfully models transition from parabolic to hyperbolic regimes

Handles formation of sharp current sheets during evolution

Compatible with long-term magneto-thermal simulations

Abstract

Over the past decade, the numerical modeling of the magnetic field evolution in astrophysical scenarios has become an increasingly important field. In the crystallized crust of neutron stars the evolution of the magnetic field is governed by the Hall induction equation. In this equation the relative contribution of the two terms (Hall term and Ohmic dissipation) varies depending on the local conditions of temperature and magnetic field strength. This results in the transition from the purely parabolic character of the equations to the hyperbolic regime as the magnetic Reynolds number increases, which presents severe numerical problems. Up to now, most attempts to study this problem were based on spectral methods, but they failed in representing the transition to large magnetic Reynolds numbers. We present a new code based on upwind finite differences techniques that can handle situations with arbitrary low magnetic diffusivity and it is suitable for studying the formation of sharp current sheets during the evolution. The code is thoroughly tested in different limits and used to illustrate the evolution of the crustal magnetic field in a neutron star in some representative cases. Our code, coupled to cooling codes, can be used to perform long-term simulations of the magneto-thermal evolution of neutron stars.