Evolution of magnetic deformation in neutron star crust

TL;DR

This paper investigates how magnetic fields evolve in neutron star crusts, revealing that magnetic deformation and ellipticity change over time, with implications for neutron star shape and magnetic field decay.

Contribution

It introduces a numerical simulation incorporating plastic flow, Ohmic dissipation, and Hall drift to study magnetic deformation evolution in neutron star crusts.

Findings

Magnetic deformation induces quadrupole shape changes.

Ellipticity generally decreases as magnetic energy diminishes.

Toroidal fields can cause the star's shape to switch from prolate to oblate.

Abstract

In this study, we examine the magnetic field evolution occurring in a neutron star crust. Beyond the elastic limit, the lattice ions are assumed to act as a plastic flow. The Ohmic dissipation, Hall drift, and bulk fluid velocity driven by the Lorentz force are considered in our numerical simulation. A magnetically induced quadrupole deformation is observed in the crust during the evolution. Generally, the ellipticity decreases as the magnetic energy decreases. In a toroidal-field-dominated model, the sign of the ellipticity changes. Namely, the initial prolate shape tends to become oblate. This occurs because the toroidal component decays rapidly on a smaller timescale than the poloidal dipole component. We find that the magnetic dipole component does not change significantly on the Hall timescale of $\sim 1$Myr for the considered simple initial models. Thus, a more complex initial model is required to study the fast decay of surface dipoles on the abovementioned timescale.