Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...349..334a&link_type=abstract
Astronomy and Astrophysics, v.349, p.334-338 (1999)
Astronomy and Astrophysics
Astrophysics
3
Accretion, Accretion Disks, Instabilities, Magnetic Fields, Turbulence, Galaxies: Magnetic Fields
Scientific paper
A nonlinear 1D dynamo model with a heuristic alpha -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 alpha -effect and oscillatory for negative alpha -effect (in the northern hemisphere). Positive alpha -effect is assumed for galaxies while there are also reasons to assume negative alpha -effect in accretion disks (northern hemisphere). For galaxies, the question is discussed whether the amplification of initial fields by differential rotation and alpha -effect is strong enough to reach the observed field strengths of order mu G. The pitch angle of the field lines, describing the spiral structure of the field, is determined. A model with pitch angles of (say) 10(deg) 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.
Arlt Rainer
Rudiger Günther
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