# Current closure through the neutron star crust

## TL;DR

This paper investigates how electric currents flow within neutron star crusts, revealing differences between millisecond and slower pulsars, and assessing the potential for crustal decoupling under strong magnetic fields.

## Contribution

It models the internal electric current paths in neutron stars, highlighting how they depend on pulsar spin periods and magnetic field strengths, and evaluates associated crustal stresses.

## Key findings

- Current reaches the crust base in millisecond pulsars.
- In slower pulsars, currents do not penetrate beyond 100 m.
- Maxwell stresses are below crustal failure threshold in typical pulsars, but may cause decoupling in magnetars.

## Abstract

Force-free pulsar magnetospheres develop a large scale poloidal electric current circuit that flows along open magnetic field lines from the neutron star to the termination shock. The electric current closes through the interior of the neutron star where it provides the torque that spins-down the star. In the present work, we study the internal electric current in an axisymmetric rotator. We evaluate the path of the electric current by requiring the minimization of internal Ohmic losses. We find that, in millisecond pulsars, the current reaches the base of the crust, while in pulsars with periods of a few seconds, the bulk of the electric current does not penetrate deeper than about $100$ m. The region of maximum spin-down torque in millisecond pulsars is the base of the crust, while in slowly spinning ones it is the outer crust. We evaluate the corresponding Maxwell stresses and find that, in typical rotation-powered radio pulsars, they are well below the critical stress that can be sustained by the crust. For magnetar-level fields, the Maxwell stresses near the surface are comparable to the critical stress and may lead to the decoupling of the crust from the rest of the stellar rotation.