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 β 0 (ratio of gas to magnetic pressure). We have considered β 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 β 0 : the radial transport of angular momentum increases with net vertical flux, achieving α ∼ 0.08 for β = 10 4 and α ≳ 1.0 for β 0 = 100, where α is the height-integrated and mass-weighted Shakura–Sunyaev parameter. A critical value lies at β 0 ∼ 10 3 : for β 0 ≳ 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 β 0 ≲ 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 β 0 ≲ 10 2 . However, the outflow is unlikely to be directly connected to a global wind: for β 0 ≳ 10 3 , the large-scale field has no permanent bending direction due to dynamo activities, while for β 0 ≲ 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.