MAGNETIC-FIELD DIFFUSION IN SELF-CONSISTENTLY TURBULENT ACCRETION DISKS

We show how the level of turbulence in accretion disks can be derived from a self-consistency requirement that the associated effective visc osity should match the instantaneous accretion rate. This method is ap plicable when turbulence has a direct energy cascade. Only limited inf ormation on the origin and properties of the turbulence, such as its i njection scale and anisotropy, is needed. The method is illustrated by considering the case of turbulence originating from the magnetic shea ring instability. The corresponding effective kinematic viscosity coef ficient is shown to scale as the 1/3 power of surface mass density at a given radius in optically thick disks, and to be describable by a Sh akura-Sunyaev law with alpha approximate to 0.04. Mass flow in disks f ed at a localized hot spot is calculated for accretion regimes driven by such turbulence, as well as passive magnetic held diffusion and dra gging. An important result of this analysis is that thin disks support ed by turbulence driven by the magnetic shearing instability, and more generally any turbulence with injection scale of order of the disk th ickness, are very low magnetic Reynolds number systems. Turbulent visc osity-driven solutions with negligible held dragging and no emission o f cold winds or jets are natural consequences of such regimes. Disks o f accreting objects that are magnetized enough to be shielded by a mag netopause, however, may not operate in their innermost regions in the magnetic shearing instability regime. The possibility therefore remain s to be explored of centrifugally driven winds emanating from such reg ions.