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E.T. Vishniac (Johns Hopkins University)
Accretion disks present two kinds of entertaining, and fundamental, puzzles for theorists to solve. First, they shine because of angular momentum transport, and the basic mechanism has been a subject of controversy for over two decades. The most likely mechanism seems to be a linear magnetic field instability (the magnetorotational, or Balbus-Hawley, instability). This leads to the second puzzle. Accretion disks generate internal magnetic fields. In fact, they represent one of the few cases where we can simulate the operation of an astrophysical dynamo. Despite this there is no wide agreement on how the dynamo operates nor how to use the simulations to construct general models of accretion disks. The success of the accretion disk simulations has been accompanied by the rise of powerful theoretical challenges to traditional mean field dynamo theory.
Here I will discuss a simple physical model of how dynamos work that overcomes these objections, and show how this model explains dynamo activity in simulations of accretion disks (as well as the lack of dynamo activity in other simulations). Real accretion disks are much more complex than the simulations, and I will discuss some of the issues involved in using these simple models to shed light on observational issues. In particular, the thermal cycling of binary accretion disks gives powerful constraints on the behavior of the disk dynamo.
Finally, I will discuss the implications of this work for other magnetic field generation in other astrophysical objects.
The author(s) of this abstract have provided an email address for comments about the abstract: ethan@pha.jhu.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.