Ohm's Law for Plasma in General Relativity and Cowling's Theorem

TL;DR

This paper derives a general-relativistic Ohm's law for plasma around neutron stars, predicts a frame-dragging induced azimuthal current that could influence magnetic field evolution, but confirms Cowling's theorem still applies in this context.

Contribution

It introduces a relativistic Ohm's law including gravitomagnetic effects and analyzes their impact on magnetic field generation around neutron stars.

Findings

Azimuthal current generated by frame-dragging effects.

Potential influence on neutron star magnetic field evolution.

Cowling's theorem remains valid in general relativity.

Abstract

The general-relativistic Ohm's law for a two-component plasma which includes the gravitomagnetic force terms even in the case of quasi-neutrality has been derived. The equations that describe the electromagnetic processes in a plasma surrounding a neutron star are obtained by using the general relativistic form of Maxwell equations in a geometry of slow rotating gravitational object. In addition to the general-relativistic effect first discussed by Khanna \& Camenzind (1996) we predict a mechanism of the generation of azimuthal current under the general relativistic effect of dragging of inertial frames on radial current in a plasma around neutron star. The azimuthal current being proportional to the angular velocity $\omega$ of the dragging of inertial frames can give valuable contribution on the evolution of the stellar magnetic field if $\omega$ exceeds $2.7\times 10^{17} (n/\sigma) \textrm{s}^{-1}$ ($n$ is the number density of the charged particles, $\sigma$ is the conductivity of plasma). Thus in general relativity a rotating neutron star, embedded in plasma, can in principle generate axial-symmetric magnetic fields even in axisymmetry. However, classical Cowling's antidynamo theorem, according to which a stationary axial-symmetric magnetic field can not be sustained against ohmic diffusion, has to be hold in the general-relativistic case for the typical plasma being responsible for the rotating neutron star.