Enhanced angular momentum transport in accretion disks

The status of our current understanding of angular momentum transport in accretion disks is reviewed. The last decade has seen a dramatic increase both in the recognition of key physical processes and in our ability to carry through direct numerical simulations of turbulent flow. Magnetic fields have at once powerful and subtle influences on the behavior of (sufficiently) ionized gas, rendering them directly unstable to free energy gradients. Outwardly descreasing angular velocity profiles are unstable. The breakdown of Keplerian rotation into MHD turbulence may be studied in some numerical detail, and key transport coefficients may be evaluated. Chandra observations of the Galactic Center support the existence of low luminosity accretion, which may ultimately prove amenable to global three-dimensional numerical simulation.