Advection/Diffusion of Large-Scale B-Field in Accretion Disks

TL;DR

This paper investigates how large-scale magnetic fields evolve in accretion disks, considering the effects of turbulence suppression at the disk surface, and finds that weak magnetic fields can persist without diffusing away.

Contribution

It introduces a model accounting for non-turbulent surface layers, showing that weak large-scale magnetic fields are retained in accretion disks contrary to previous assumptions.

Findings

Weak magnetic fields do not diffuse away as previously thought.

Surface layers' non-turbulence allows magnetic field retention.

Stationary solutions support persistent large-scale magnetic fields.

Abstract

Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical ($z$) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. The stationary solutions we find indicate that a weak ($\beta > 1$) large-scale field does not diffuse away as suggested by earlier work.