Cylindrical anisotropic α 2 dynamos
Résumé
We explore the influence of geometry variations on the structure and the time-dependence of the magnetic field that is induced by kinematic α2 dynamos in a finite cylinder. The dynamo action is due to an anisotropic α effect which can be derived from an underlying columnar flow. The investigated geometry variations concern, in particular, the aspect ratio of height to radius of the cylinder, and the thickness of the annular space to which the columnar flow is restricted. Motivated by the quest for laboratory dynamos which exhibit Earth-like features, we start with modifications of the Karlsruhe dynamo facility. Its dynamo action is reasonably described by an α2 mechanism with anisotropic α tensor. We find a critical aspect ratio below which the dominant magnetic field structure changes from an equatorial dipole to an axial dipole. Similar results are found for α2 dynamos working in an annular space when a radial dependence of α is assumed. Finally, we study the effect of varying aspect ratios of dynamos with an α tensor depending both on radial and axial coordinates. In this case only dominant equatorial dipoles are found and most of the solutions are oscillatory, contrary to all previous cases where the resulting fields are steady.