1MAG (ENS/IPGP), LRA, École Normale Supérieure,
24 rue Lhomond, 75252 Paris Cedex 05, France
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.
Key words: dynamo / magnetohydrodynamics (MHD) / magnetic fields / Sun: dynamo