LOCAL STUDY OF ACCRETION DISKS WITH A STRONG VERTICAL MAGNETIC FIELD: MAGNETOROTATIONAL INSTABILITY AND DISK OUTFLOW

We perform three-dimensional, vertically-stratified, local shearing-box ideal MHD simulations of the magnetorotational instability (MRI) that include a net vertical magnetic flux, which is characterized by midplane plasma beta(0) (ratio of gas to magnetic pressure). We have considered beta(0) = 10(2), 10(3), and 10(4), and in the first two cases the most unstable linear MRI modes are well resolved in the simulations. We find that the behavior of the MRI turbulence strongly depends on beta(0): the radial transport of angular momentum increases with net vertical flux, achieving alpha similar to 0.08 for beta = 10(4) and alpha greater than or similar to 1.0 for beta(0) = 100, where alpha is the height-integrated and mass-weighted Shakura-Sunyaev parameter. A critical value lies at beta(0) similar to 10(3): for beta(0) greater than or similar to 10(3), the disk consists of a gas pressure dominated midplane and a magnetically dominated corona. The turbulent strength increases with net flux, and angular momentum transport is dominated by turbulent fluctuations. The magnetic dynamo that leads to cyclic flips of large-scale fields still exists, but becomes more sporadic as net flux increases. For beta(0) less than or similar to 10(3), the entire disk becomes magnetically dominated. The turbulent strength saturates, and the magnetic dynamo is fully quenched. Stronger large-scale fields are generated with increasing net flux, which dominates angular momentum transport. A strong outflow is launched from the disk by the magnetocentrifugal mechanism, and the mass flux increases linearly with net vertical flux and shows sign of saturation at beta(0) less than or similar to 10(2). However, the outflow is unlikely to be directly connected to a global wind: for beta(0) greater than or similar to 10(3), the large-scale field has no permanent bending direction due to dynamo activities, while for beta(0) less than or similar to 10(3), the outflows from the top and bottom sides of the disk bend towards opposite directions, inconsistent with a physical disk wind geometry. Global simulations are needed to address the fate of the outflow.