Angular Momentum Transport in Low Mass Accretion Disks

We consider 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 α model is probably not a good description of real disks. Here we briefly review a few of the more promising models. These include 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 α−Ω dynamo. The azimuthal magnetic field produced in this way is unstable to a magnetic shearing instability (MSI), which drives nearly isotropic turbulent eddys with typical fluid velocities of ∼ VA, where VA is the local Alfvén speed. The scale of the eddys is ∼ VA/Ω, where Ω is the local rotational frequency. This turbulence leads to a saturation of the dynamo when VA ∼ (H/r) cs, where H is the halfthickness of the disk, cs is the local sound speed, and r is the radial coordinate. This gives rise to an effective dimensionless viscosity coefficient ∼ (H/r) and vertical and radial diffusion coefficients which are ∼ (H/r)Hcs. The resulting vertical diffusion of entropy will have a substantial effect on detailed models of vertical structure in accretion disks. Viscous and thermal instabilities of very hot disks, those dominated by radiation pressure and electron scattering, are substantially moderated in this model. We note that the MSI largely suppresses the Parker instability in accretion disks.