  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 omega-effect, i.e. by the stretching of field lines due to 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. For a resulting
oscillatory model, dynamo coefficients have been computed with the help of the
so-called test-field method. Subsequently, these coefficients have been used in
a mean-field calculation in order to explore the underlying dynamo mechanism.
Results: Although the rather strong differential rotation present in this model
influences the magnetic field, the omega-effect alone is not responsible for
its cyclic time variation. If the omega-effect is suppressed, the resulting
alpha^2-dynamo remains oscillatory. Surprisingly, the corresponding alpha-omega
dynamo leads to a non-oscillatory magnetic field. Conclusions: The assumption
of an alpha-omega mechanism does not explain the occurrence of magnetic cycles
satisfactorily.
