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Astron. Astrophys. 356, 1141-1148 (2000)

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

U. Ziegler and G. Rüdiger

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received 31 January 2000 / Accepted 1 March 2000

Abstract

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 [FORMULA] is estimated to [FORMULA] 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-[FORMULA] 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-) [FORMULA]-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

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000
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