Turbulent dynamos with shear and fractional helicity

P. J. Käpylä; A. Brandenburg

doi:10.1088/0004-637x/699/2/1059

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arXiv · 0810.2298

Turbulent dynamos with shear and fractional helicity

P. J. Käpylä·A. Brandenburg

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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 modelled in terms of an αΩdynamo. 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 (Rm) 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 Rm.

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P. J. Käpylä, A. Brandenburg. 2009-07-29. Turbulent dynamos with shear and fractional helicity. https://doi.org/10.1088/0004-637x%2F699%2F2%2F1059

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