abstract
- We review mechanisms for the transport of angular momentum in accretion disks that are low mass, in the sense that the gravitational forces produced by the material in these disks has a negligible effect on disk dynamics. There is no established consensus on how this transport takes place. We note that for phenomenological reasons the traditional $\alpha$ model is probably not a good description of real disks. We discuss models in which angular momentum transport is driven by shocks, and by magnetic field instabilities. The latter is more promising, but requires a dynamo. We note that the direction of angular momentum transport due to convection in a conducting disk is not known as competing mechanisms are at work. We briefly discuss a number of possible dynamo mechanisms, and their problems. We then give a detailed exposition of the internal wave driven dynamo model, in which internal waves excited at large radii drive an $\alpha-\Omega$ dynamo.