Dynamos and angular momentum transport in accretion disks

Abstract

The transport of angular momentum in astrophysical disks is one of the major issues in modern astrophysics. Here, recent work [Astrophys. J. 347, 435 (1989); 365, 648 (1990)] will be reviewed that suggests that internal waves, analogous to deep ocean waves, play a critical role in transporting angular momentum in neutral disks and generating a magnetic dynamo in ionized disks. Previously, it was shown that low-frequency, slightly nonaxisymmetric (‖m‖=1) waves in thin accretion disks could penetrate to small radii with a unique amplitude because of nonlinear saturation. Here, the ability of these waves to drive an α-Ω dynamo in a disk of thickness H and radius r and keplerian rotational frequency Ω(r)∝r−3/2 is examined. The asymmetry in the wave distribution that creates a nonzero helicity follows from the fact that the fundamental waves all have a positive angular momentum flux. As a result, there will be a large-scale magnetic field driven by an α-Ω dynamo. It is also likely that small-scale fields, driven by higher-order wave modes, will contribute significantly to the local value of BrBφ. It is argued that the magnetic field saturates when its pressure is comparable to the thermal pressure and a crude model of the nonlinear transfer of power to small-scale turbulence is presented. The dynamo process creates a large-scale, axisymmetric toroidal field with Br∼(H/r)3/2Bφ. Smaller-scale waves create small-scale fields with a maximum brbφ∼(H/r)6/5P. In this model, viscous and thermal instabilities in radiation pressure dominated, and electron scattering regions in accretion disks appear to be substantially suppressed.