The magnetic shear instability is reviewed numerically in the local box app
roximation for a Kepler disk. Special emphasis is laid on the relation betw
een the viscosity-alpha and dynamo-alpha in case a mean magnetic held is ge
nerated.
Self-sustaining 'turbulence' is initiated by the instability which acts sim
ultaneously as dynamo and efficient outward transporter for angular momentu
m. The Shakura-Sunyaev parameter alpha ss is estimated to approximate to 1.
5 10(-2) for an adiabatic disk model, and the contribution from the Maxwell
stress dominates over that of the Reynolds stress by a factor of 4.
In case of stress-free, normal-B vertical boundary conditions, a non-zero m
ean magnetic field mainly oriented in azimuthal direction is generated. Thi
s mean field turns out time-dependent in a quasi-periodic manner. Box reson
ance oscillations in the horizontal velocities for a limited time lead to a
n enhanced, violently fluctuating Reynolds stress associated with a reduced
magnetic activity. The resulting (dynamo-) alpha-effect is negative in the
upper disk plane and positive in the lower disk plane, it is small and hig
hly noisy.