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We perform 3D vertically-stratified local shearing-box ideal MHD simulationsof the magnetorotational instability (MRI) that include a net vertical magneticflux, which is characterized by beta_0 (ratio of gas pressure to magneticpressure of the net vertical field at midplane). We have consideredbeta_0=10^2, 10^3 and 10^4 and in the first two cases the most unstable linearMRI modes are well resolved in the simulations. We find that the behavior ofthe MRI turbulence strongly depends on beta_0: The radial transport of angularmomentum increases with net vertical flux, achieving alpha=0.08 for beta_0=10^4and alpha>1.0 for beta_0=100, where alpha is the Shakura-Sunyaev parameter. Acritical value lies at beta_0=10^3: For beta_0>10^3, the disk consists of a gaspressure dominated midplane and a magnetically dominated corona. The turbulentstrength increases with net flux, and angular momentum transport is dominatedby turbulent fluctuations. The magnetic dynamo that leads to cyclic flips oflarge-scale fields still exists, but becomes more sporadic as net fluxincreases. For beta_0<10^3, the entire disk becomes magnetic dominated. Theturbulent strength saturates, and the magnetic dynamo is quenched. Strongerlarge-scale fields are generated with increasing net flux, which dominatesangular momentum transport. A strong outflow is launched from the disk by themagnetocentrifugal mechanism, and the mass flux increases linearly with netvertical flux and shows sign of saturation at beta_0=10^2. However, the outflowis unlikely to be directly connected to a global wind: for beta_0>10^3, thelarge-scale field has no permanent bending direction due to dynamo activities,while for beta_0<10^3, the outflows from the top and bottom sides of the diskbend towards opposite directions, inconsistent with a physical disk windgeometry. Global simulations are needed to address the fate of the outflow.

Local Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow