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Astron. Astrophys. 349, 334-338 (1999)


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Accretion-disk dynamo models with dynamo-induced alpha-effect

R. Arlt and G. Rüdiger

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

Received 24 February 1999 / Accepted 21 July 1999

Abstract

A nonlinear 1D dynamo model with a heuristic [FORMULA]-ansatz for magnetic field-induced turbulence is considered to be growing for weak fields and suppressed by strong fields. The amplification of initial magnetic fields is regarded in accretion disks like both Kepler disks and galaxies. The solutions for the magnetic field are symmetric with respect to the equator, they are steady for positive [FORMULA]-effect and oscillatory for negative [FORMULA]-effect (in the northern hemisphere). Positive [FORMULA]-effect is assumed for galaxies while there are also reasons to assume negative [FORMULA]-effect in accretion disks (northern hemisphere). For galaxies, the question is discussed whether the amplification of initial fields by differential rotation and [FORMULA]-effect is strong enough to reach the observed field strengths of order µG. The pitch angle of the field lines, describing the spiral structure of the field, is determined. A model with pitch angles of (say) 10o as observed in galaxies reaches a magnetic amplification factor of only 300. Only a few rotation times are necessary to establish the final configuration. Once established, the highly nonlinear dynamo never decays.

Key words: accretion, accretion disks – instabilities – magnetic fields – turbulence – galaxies: magnetic fields

Send offprint requests to: G. Rüdiger

© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999

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