  We investigate the effect of shear viscosity and Ohmic resistivity 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, Pm = viscosity/resistivity, on the turbulence is
captured by ~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
|z| < 2 H. Above |z| = 2 H, 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, on the order of cs H/1000, where cs is the
sound speed, MRI turbulence within 2 H of the mid-plane undergoes periods of
resistive decay followed by regrowth. This regrowth is caused by amplification
of toroidal field via the dynamo. This process results in large amplitude
variability in the stress on 10 to 100 orbital timescales, which may have
relevance for partially ionized disks that are observed to have high and low
accretion states.
