On the theory of the geomagnetic dynamo based on mean field electrodynamics

A survey is given on the present state of the geomagnetic dynamo theory which is based on the electrodynamics of mean fields in electrically conducting turbulent media. Models of both the ²-type and the ω-type are taken into consideration, i.e. models where the -effect alone, or together with a differential rotation, gives rise to the regeneration of magnetic fields. Results of special investigations are given and discussed. Models of the ²-type apply to the Earth provided that it shows no noticeable differential rotation. It is pointed out that such models can explain not only the generation of a magnetic field with an axisymmetric dipole-like structure, but also the occurrence of non-axisymmetric components and their westward drift. If, however, a noticeable differential rotation exists, models of the ω-type have to be envisaged. Such models also allow an understanding of the existence of a magnetic field with an axisymmetric dipole-like structure, but they do not provide for a straightforward interpretation of the occurrence and behaviour of the non-axisymmetric components.