Magnetic viscosity in weakly ionized protostellar discs

It has been shown that a self-consistent magnetic dynamo can operate in a fully ionized accretion disc and so provide an effective (magnetic) viscosity. Parker's instability (magnetic buoyancy) in weakly ionized protostellar discs is considered. Combined with the Balbus-Hawley instability in a similar medium, magnetic reconnection and the dynamo model are generalized for weakly ionized discs. The increased rate of reconnection and the decreased growth rate of the Balbus-Hawley instability from increased azimuthal fields have opposite effects. It is found that the Shakura-Sunyaev viscosity parameter does not change for very weakly ionized protostellar accretion discs (alpha(SS)similar to 0.4).