RESISTIVITY-DRIVEN STATE CHANGES IN VERTICALLY STRATIFIED ACCRETION DISKS

We investigate the effect of shear viscosity, nu, and Ohmic resistivity, eta, on the magnetorotational instability (MRI) in vertically stratified accretion disks through a series of local simulations with the Athena code. First, we use a series of unstratified simulations to calibrate physical dissipation as a function of resolution and background field strength; the effect of the magnetic Prandtl number, P-m = nu/eta, on the turbulence is captured by similar to 32 grid zones per disk scale height, H. In agreement with previous results, our stratified disk calculations are characterized by a subthermal, predominately toroidal magnetic field that produces MRI-driven turbulence for vertical bar z vertical bar less than or similar to 2H. Above vertical bar z vertical bar similar to 2H, the magnetic pressure dominates and the field is buoyantly unstable. Large-scale radial and toroidal fields are also generated near the mid-plane and subsequently rise through the disk. The polarity of this mean field switches on a roughly 10 orbit period in a process that is well modeled by an alpha-Omega dynamo. Turbulent stress increases with Pm but with a shallower dependence compared to unstratified simulations. For sufficiently large resistivity, eta similar to c(s)H/1000, where c(s) is the sound speed, MRI turbulence within 2H of the mid-plane undergoes periods of resistive decay followed by regrowth. This regrowth is caused by amplification of the toroidal field via the dynamo. This process results in large amplitude variability in the stress on 10-100 orbital timescales, which may have relevance for partially ionized disks that are observed to have high- and low-accretion states.