paper

A Magnetic Alpha-Omega Dynamo in Active Galactic Nuclei Disks: II. Magnetic Field Generation, Theories and Simulations

arXiv:astro-ph/0611188 · doi:10.1086/510735

Abstract

It is shown that a dynamo can operate in an Active Galactic Nuclei accretion disk due to the Keplerian shear and due to the helical motions of expanding and twisting plumes of plasma heated by many star passages through the disk. Each plume rotates a fraction of the toroidal flux into poloidal flux, always in the same direction, through a finite angle, and proportional to its diameter. The predicted growth rate of poloidal magnetic flux, based upon two analytic approaches and numerical simulations, leads to a rapid exponentiation of a seed field, \sim 0.1 to \sim 0.01 per Keplerian period of the inner part of the disk. The initial value of the seed field may therefore be arbitrarily small yet reach, through dynamo gain, saturation very early in the disk history. Because of tidal disruption of stars close to the black hole, the maximum growth rate occurs at a radius of about 100 gravitational radii from the central object. The generated mean magnetic field, a quadrupole field, has predominantly even parity so that the radial component does not reverse sign across the midplane. The linear growth is predicted to be the same by each of the following three theoretical analyses: the flux conversion model, the mean field approach, and numerical modeling. The common feature is the conducting fluid flow, considered in companion Paper I (Pariev and Colgate 2006) where two coherent large scale flows occur naturally: the differential winding of Keplerian motion and differential rotation of expanding plumes.

arXiv comment70 pages, 21 figure, accepted for publication in ApJ

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