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Title:
Turbulent Dynamos with Shear and Fractional Helicity
Authors:
Käpylä, Petri J.; Brandenburg, Axel
Affiliation:
AA(Observatory, University of Helsinki, P.O. Box 14, FI-00014 University of Helsinki, Finland), AB(NORDITA, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden)
Publication:
The Astrophysical Journal, Volume 699, Issue 2, pp. 1059-1066 (2009). (ApJ Homepage)
Publication Date:
07/2009
Origin:
IOP
Astronomy Keywords:
MHD, turbulence
DOI:
10.1088/0004-637X/699/2/1059
Bibliographic Code:
2009ApJ...699.1059K

Abstract

Dynamo action owing to helically forced turbulence and large-scale shear is studied using direct numerical simulations. The resulting magnetic field displays propagating wave-like behavior. This behavior can be modeled in terms of an αΩ dynamo. In most cases super-equipartition fields are generated. By varying the fraction of helicity of the turbulence the regeneration of poloidal fields via the helicity effect (corresponding to the α-effect) is regulated. The saturation level of the magnetic field in the numerical models is consistent with a linear dependence on the ratio of the fractional helicities of the small and large-scale fields, as predicted by a simple nonlinear mean-field model. As the magnetic Reynolds number (Re M ) based on the wavenumber of the energy-carrying eddies is increased from 1 to 180, the cycle frequency of the large-scale field is found to decrease by a factor of about 6 in cases where the turbulence is fully helical. This is interpreted in terms of the turbulent magnetic diffusivity, which is found to be only weakly dependent on the Re M .
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