Angular momentum transport and dynamo-effect in stratified, weakly magnetic disks

The magnetic shear instability is reviewed numerically in the local box approximation for a Kepler disk. Special emphasis is laid on the relation between the viscosity–alpha and dynamo–alpha in case a mean magnetic field is generated. Self-sustaining ‘turbulence’ is initiated by the instability which acts simultaneously as dynamo and efficient outward transporter for angular momentum. The Shakura-Sunyaev parameter αSS is estimated 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 mean magnetic field mainly oriented in azimuthal direction is generated. This mean field turns out timedependent in a quasi-periodic manner. Box resonance oscillations in the horizontal velocities for a limited time lead to an enhanced, violently fluctuating Reynolds stress associated with a reduced magnetic activity. The resulting (dynamo-) α-effect is negative in the upper disk plane and positive in the lower disk plane, it is small and highly noisy.