Combined magnetic field evolution in neutron star cores and crusts: Ambipolar diffusion, Hall effect and Ohmic dissipation

TL;DR

This study presents simulations of the combined magnetic field evolution in neutron star cores and crusts, highlighting the interplay of ambipolar diffusion, Hall effect, and Ohmic dissipation, and their impact on magnetic decay and symmetry breaking.

Contribution

It provides the first detailed simulation of simultaneous magnetic field evolution in neutron star cores and crusts, incorporating all three effects under axial symmetry.

Findings

Hall effect generates toroidal fields from poloidal fields in the crust.

Ambipolar diffusion tends to restore north-south symmetry broken by Hall effect.

Magnetic field decay is enhanced by ambipolar diffusion, increasing heat generation.

Abstract

Neutron star magnetic field evolution is mediated through the Hall effect and Ohmic dissipation in the crust while ambipolar diffusion is taking place in the core. These effects have been studied in detail in either part of the star, however, their combined, simultaneous evolution and interplay has not been explored in detail yet. Here, we present simulation results of the simultaneous evolution of the magnetic field in the core due to ambipolar diffusion and the crust due to Hall effect and Ohmic decay, under the assumption of axial symmetry. We find that a purely poloidal field generates a toroidal field in the crust, due to the Hall effect, that sinks into the core. A purely toroidal field remains toroidal and spreads into the core and the crust. Finally, for a mixed poloidal-toroidal field, the north-south symmetry is broken due to the Hall effect in the crust, however, ambipolar diffusion, tends to restore it. We examine the role of ambipolar diffusion to the magnetic field decay and we compare the rate of the conversion of magnetic field energy into heat, finding that it enhances the magnetic field decay in neutron stars.