Magnetic Field Evolution in Neutron Star Crusts: Beyond the Hall Effect

TL;DR

This paper reviews recent advances in understanding the magnetic field evolution in neutron star crusts, emphasizing effects beyond the traditional Hall effect, including crust failure, temperature variations, and battery terms, which impact neutron star observations.

Contribution

It introduces new considerations of crust failure and temperature effects on magnetic field evolution beyond the Hall effect framework.

Findings

Crust failure influences magnetic field evolution.

Temperature variations affect magnetic field structure.

Battery effects can activate in weakly magnetized neutron stars.

Abstract

Neutron stars host the strongest magnetic fields that we know of in the Universe. Their magnetic fields are the main means of generating their radiation, either magnetospheric or through the crust. Moreover, the evolution of the magnetic field has been intimately related to explosive events of magnetars, which host strong magnetic fields, and their persistent thermal emission. The evolution of the magnetic field in the crusts of neutron stars has been described within the framework of the Hall effect and Ohmic dissipation. Yet, this description is limited by the fact that the Maxwell stresses exerted on the crusts of strongly magnetised neutron stars may lead to failure and temperature variations. In the former case, a failed crust does not completely fulfil the necessary conditions for the Hall effect. In the latter, the variations of temperature are strongly related to the magnetic field evolution. Finally, sharp gradients of the star's temperature may activate battery terms and alter the magnetic field structure, especially in weakly magnetised neutron stars. In this review, we discuss the recent progress made on these effects. We argue that these phenomena are likely to provide novel insight into our understanding of neutron stars and their observable properties.