The alpha-effect in galaxies is highly anisotropic

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Abstract
For the design of galactic dynamo models a knowledge of the complete α-tensor is needed. We determine here its symmetric part with the idea that a homogeneous and isotropic field of explosive forces (created by supernovae or stellar winds) drives a turbulent flow which becomes helical under the influence of rotation and density stratification. Already in the simplest approach that ignores density fluctuations and magnetic diffusivity, and assumes weak stratification and slow rotation, the vertical α-effect exceeds the horizontal one by a factor of 5, α∥/α⊥ ≈ 5. This trend is even amplified if, for small Prandtl number in a more realistic approach, ‘‘negative buoyancy'’ in the galactic isothermal layer is invoked. If, in an alternative model, the turbulence is replaced by random waves the horizontal α⊥ exactly vanishes but not α∥.

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