Dynamos and angular momentum transport in accretion disks
Journal Articles
Overview
Research
Identity
Additional Document Info
View All
Overview
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.
