Speaker
Markus Gellert
Description
Helical magnetic background fields with adjustable pitch
angle are imposed on a conducting fluid in a differentially
rotating cylindrical container. The small-scale kinetic and
current helicities are calculated for various field
geometries, and shown to have the opposite sign as the
helicity of the large-scale field. These helicities and also
the corresponding $\alpha$-effect scale with the current
helicity of the background field. The $\alpha$-tensor is
highly anisotropic as the components $\alpha_{\phi\phi}$ and
$\alpha_{zz}$ have opposite signs. The amplitudes of the
azimuthal $\alpha$-effect computed with the cylindrical 3D
MHD code are so small that the operation of an
$\alpha\Omega$ dynamo on the basis of the current-driven,
kink-type instabilities of toroidal fields is highly
questionable. In any case the low value of the
$\alpha$-effect would lead to very long growth times of a
dynamo in the radiation zone of the Sun and early-type stars
of the order of mega-years.