  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.
