Transport in accretion disks

Astrophysical accretion disks are powered by the release of gravitational p otential energy as gas spirals down onto a compact star or black hole. The dynamics and evolution of accretion disks depend upon how angular momentum is transported outward from one fluid element to another. The nature of thi s process was unclear for many years. Since the early 1990s, however, consi derable progress has been made in understanding how turbulence arises and t ransports angular momentum in astrophysical accretion disks. Accretion disk s are generally highly conducting plasmas; the equations governing their ev olution are those of ideal magnetohydrodynamics. Although a hydrodynamical disk would be locally stable, the combination of a weak subthermal magnetic field and outwardly decreasing differential rotation rapidly generates mag netohydrodynamical turbulence via a remarkably simple linear instability. T hus, turbulent accretion disks are fundamentally magnetohydrodynamical in n ature. (C) 1999 American Institute of Physics. [S1070-664X(99)03512-0].