U. Ziegler et G. Rudiger, Shear rate dependence and the effect of resistivity in magneto-rotationally unstable, stratified disks, ASTRON ASTR, 378(2), 2001, pp. 668-678
We study the effect of resistivity and variation of shear rate, q = -d log
Omega /dR, on the magneto-rotational instability in stratified disks. Speci
al attention is paid to dynamo alpha -action and transport properties of th
e flow. The 3D magnetohydrodynamical simulations are performed in the local
shearing-box approximation employing the code NIRVANA. Due to the chosen t
ype of vertical boundary condition a mean azimuthal magnetic field is gener
ated which is related to dynamo alpha -action. The corresponding alpha -par
ameter is negative (positive) in the northern (southern) disk hemisphere. T
his 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 wit
h 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 signif
icant influence on the magnetic energy level of the final state. It is redu
ced for decreasing magnetic Reynolds number, Re-m, as one might expect. How
ever, the magneto-rotational instability is able to sustain an angular mome
ntum transport of sufficient amount against diffusion even for the most dif
fusive case we considered, with Re-m = 200. The Shakura-Sunyaev parameter i
n this case is roughly 1/3 of that of the non-resistive model with the domi
nant contribution provided by the Reynolds stress rather than Maxwell stres
s. The dynamo alpha -effect is found to vanish for strong dissipation.