Shear rate dependence and the effect of resistivity in magneto-rotationally unstable, stratified disks

We study the effect of resistivity and variation of shear rate, q = -d log Omega /dR, on the magneto-rotational instability in stratified disks. Special attention is paid to dynamo alpha -action and transport properties of the flow. The 3D magnetohydrodynamical simulations are performed in the local shearing-box approximation employing the code NIRVANA. Due to the chosen type of vertical boundary condition a mean azimuthal magnetic field is generated which is related to dynamo alpha -action. The corresponding alpha -parameter is negative (positive) in the northern (southern) disk hemisphere. This result is independent of q ranging between 0.6 and 1.5 and independent of the amount of diffusivity. The magnitude of alpha typically declines with decreasing q. We suggest a natural limit in the vicinity of q = 0.6 below which alpha -action may be suppressed completely. Resistivity has a significant influence on the magnetic energy level of the final state. It is reduced for decreasing magnetic Reynolds number, Re-m, as one might expect. However, the magneto-rotational instability is able to sustain an angular momentum transport of sufficient amount against diffusion even for the most diffusive case we considered, with Re-m = 200. The Shakura-Sunyaev parameter in this case is roughly 1/3 of that of the non-resistive model with the dominant contribution provided by the Reynolds stress rather than Maxwell stress. The dynamo alpha -effect is found to vanish for strong dissipation.