The Cosmic Battery in Astrophysical Accretion Disks

TL;DR

This paper uses numerical simulations to confirm the Cosmic Battery mechanism in accretion disks around black holes, showing how magnetic fields grow via radiation-induced currents and are affected by turbulence and magnetic Prandtl number.

Contribution

It provides realistic simulations of the Cosmic Battery in ADAF accretion flows, confirming magnetic field growth and identifying conditions that suppress the mechanism.

Findings

Inner magnetic loops are advected inward, contributing to field growth.

Outer loops diffuse outward, balancing the inward advection.

The mechanism is suppressed when the magnetic Prandtl number exceeds a critical value.

Abstract

The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows-ADAF. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.