Oscillatory dynamos and their induction mechanisms

TL;DR

This paper investigates the dynamo mechanisms behind oscillatory magnetic fields, revealing that the classical alpha-omega model alone cannot fully explain the cyclic behavior observed in such systems.

Contribution

The study demonstrates that in an oscillatory dynamo model, the alpha^2 mechanism can produce cycles, and the alpha-omega model does not necessarily lead to oscillations, challenging traditional assumptions.

Findings

Omega-effect alone does not cause cyclic magnetic variations.

Alpha^2 dynamo can be oscillatory without omega-effect.

Alpha-omega dynamo may produce non-oscillatory fields.

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 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.