FLUCTUATIONS, DISSIPATION AND TURBULENCE IN ACCRETION DISKS

In this paper, we relate dissipational processes in accretion discs to large-scale correlated fluctuations in the velocity and magnetic fiel d components. The radialazimuthal (Rphi) component of the correlation stress tensor for velocity and magnetic fluctuations is responsible fo r transport within the disc. Under steady conditions, the Rphi compone nt of the stress tensor scales with disc radius R and surface density SIGMA as 1/(R3/2SIGMA). This component of the stress tensor can also b e related in a simple way to the local radiative flux density. This la tter relation is observationally sensitive to surface irradiation, but the former scaling law is expected to be very robust. Both results ar e examples of fluctuation-dissipation relations for accretion discs. E clipse mapping and Doppler tomography techniques in cataclysmic variab les and X-ray binary systems may soon be able to test these types of t heoretical predictions. Molecular viscosity serves only as an energy s ink, not as a source of any turbulent transport, thermalizing the high -wavenumber end of the power spectrum. The same is true of microscopic resistivity with respect to magnetic field fluctuations. This implies that an explicit representation for the viscosity or resistivity is n ot essential for large-scale numerical modelling of accretion disc tur bulence.