Viscosity Mechanisms in Accretion Disks
  The self-sustained turbulence which develops in magnetized accretion disks is suppressed in the weakly-ionized, quiescent disks of close binary stars. Because accretion still proceeds during quiescence, another viscosity mechanism operates in these systems. An anticorrelation of the recurrence times of SU UMa dwarf novae with their mass ratio supports spiral waves or shock-waves tidally induced by the companion star as the main process responsible for accretion in the quiescent disks. Other weakly-ionized gaseous disks in systems lacking a massive companion have to rely on yet another transport mechanism or they could be essentially passive. 
Kristen Menou (Princeton Univ.)