| Abstract: | The paper supplements an earlier one on the mean-field approach to spherical kinematic dynamo models (Rädler 1980a) by results of numerical investigations. A number of dynamo models working on the basis of the α2-mechanism are considered. Cases of pure α2-mechanism are studied, which includes only the simplest form of α-effect and no other induction effect, as well as cases with several additional effects due to fluctuating or mean motions. By the pure α2-mechanism axisymmetric and non-axisymmetric fields, can be excited and maintained with nearly equal ease. Part of the additional induction effects, however, clearly favour axisymmetric fields, and others non-axisymmetric fields. The non-axisymmetric fields are waves which travel in azimuthal direction, eastward or westward, depending on the models. For special dynamo models the transition from α2 to αω-mechanism and properties of the latter are investigated. The results support the presumption that the αω-mechanism is able to maintain only axisymmetric but never non-axisymmetric fields. Conditions for the occurrence of non-oscillatory or oscillatory fields are discussed, and again the influence of additional induction effects is studied. There are further presented a model with βω-mechanism maintaining an axisymmetric non-oscillatory field, and models with two kinds of δω-mechanisms allowing axisymmetric non-oscillatory and oscillatory fields. Some ideas concerning dynamo models for the Earth, the Sun and magnetic stars are discussed. It seems possible to construct dynamo models for the Earth, on the basis of the α2-mechanism which explain not only the presence of a magnetic field with a strong dipole part but also the inclination of the dipole axis against the axis of rotation, the occurrence of higher multipoles and the westward drift of the non-axisymmetric parts. Models with αω, βω or δω-mechanism, which have to be considered in the case of a strong differential rotation inside the core, provide an explanation at first only of the axisymmetric parts of the field, and the non-axisymmetric parts have then to be interpreted, for example, as MAC-waves. As far as dynamo models for the Sun are concerned, in addition to the possibility of an αω-mechanism also that of a βω or δω-mechanism is discussed, which, however, does not look not very promising. In the models developed so far, which work with the αω-mechanism, only a few of the induction effects of fluctuating motions have been included; it seems necessary to investigate also influences of other effects. The sectorial structure of the solar magnetic field can hardly be understood in terms of the traditional mean-field concept. The magnetic stars possess fields which strongly deviate from symmetry with respect to the axis of rotation. The occurrence of such fields seems understandable only if there is no noticeable differential rotation. They can be maintained by an α2-mechanism but hardly by αω, βω or δω-mechanisms. |
