A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks

The evolution of the magnetorotational instability (MRI) during the transition from outburst to quiescence in a dwarf nova disk is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted for the analysis, so that the efficiency of angular momentum transport is studied in a small local patch of the disk: this is usually referred as to a one-zone model. To take account of the low ionization fraction of the disk, the induction equation includes both ohmic dissipation and the Hall effect. We induce a transition from outburst to quiescence by an instantaneous decrease of the temperature. The evolution of the MRI during the transition is found to be very sensitive to the temperature of the quiescent disk. As long as the temperature is higher than a critical value of about 2000 K, MHD turbulence and angular momentum transport is sustained by the MRI. However, MHD turbulence dies away within an orbital time if the temperature falls below this critical value. In this case, the stress drops off by more than 2 orders of magnitude, and is dominated by the Reynolds stress associated with the remnant motions from the outburst. The critical temperature depends slightly on the distance from the central star and the local density of the disk. Subject headings: accretion, accretion disks — diffusion — instabilities — MHD — novae, cataclysmic variables — turbulence