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Angular momentum transport and dynamo-effect in stratified, weakly magnetic disks
U. Ziegler and
Received 31 January 2000 / Accepted 1 March 2000
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 is estimated to 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- vertical boundary conditions, a non-zero mean magnetic field mainly oriented in azimuthal direction is generated. This mean field turns out time-dependent 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-) -effect is negative in the upper disk plane and positive in the lower disk plane, it is small and highly noisy.
Key words: Magnetohydrodynamics (MHD) instabilities turbulence magnetic fields
Send offprint requests to: U. Ziegler
Online publication: April 17, 2000