Transport in accretion disks

Astrophysical accretion disks are powered by the release of gravitational potential 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 this process was unclear for many years. Since the early 1990s, however, considerable progress has been made in understanding how turbulence arises and transports angular momentum in astrophysical accretion disks. Accretion disks are generally highly conducting plasmas; the equations governing their evolution 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 magnetohydrodynamical turbulence via a remarkably simple linear instability. Thus, turbulent accretion disks are fundamentally magnetohydrodynamical in nature.