Oscillatory dynamos and their induction mechanisms

¹ MAG (ENS/IPGP), LRA, École Normale Supérieure, 24 rue Lhomond, 75252 Paris Cedex 05, France
e-mail: martin@schrinner.eu
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 20 December 2010
Accepted: 5 April 2011

Abstract

Context. Large-scale magnetic fields resulting from hydromagnetic dynamo action may differ substantially in their time dependence. Cyclic field variations, characteristic for the solar magnetic field, are often explained by an important Ω-effect, i.e., by the stretching of field lines because of strong differential rotation.

Aims. The dynamo mechanism of a convective, oscillatory dynamo model is investigated.

Methods. We solve the MHD-equations for a conducting Boussinesq fluid in a rotating spherical shell. We computed the dynamo coefficients for the resulting oscillatory model with the help of the so-called test-field method. Subsequently, these coefficients were used in a mean-field calculation to explore the underlying dynamo mechanism.

Results. The oscillatory dynamo model we consider is an α²Ω one. Although the fairly strong differential rotation of this model influences the magnetic field, the Ω-effect alone is not responsible for its cyclic time variation. If the Ω-effect is suppressed, the resulting α²-dynamo remains oscillatory. Surprisingly, the corresponding αΩ-dynamo leads to a non-oscillatory magnetic field.

Conclusions. The assumption of an αΩ-mechanism does not explain the occurrence of magnetic cycles satisfactorily.