Planetary Dynamos

The theory of planetary dynamos and its applications to observed phenomena of planetary magnetism are outlined. It is generally accepted that convection flows driven by thermal or compositional buoyancy are the most likely source for the sustenance of global planetary magnetic fields. While the existence of dynamos in electrically conducting fluid planetary cores provides constraints on properties of the latter, the lack of knowledge about time dependences of the magnetic fields and about their toroidal components together with the restricted parameter regions accessible to theory have prevented so far a full understanding of the phenomena of planetary magnetism. While the Earth's magnetism has been studied for centuries starting with the first scientific monograph of Gilbert (1600), the question of the magnetism of other planets had received scant attention until recently because of the lack of relevant observations. Only in 1955 clear evidence for the existence of a planetary magnetic field other than the geomagnetic one was obtained through the observation of the Jovian decametric radio waves (Burke and Franklin, 1955). Since it had been more or less accepted until the end of the 19-th century that geomagnetism arises from the remnant magnetiza-tion of the Earth similar properties may have been assumed for the other terrestrial planets and the Moon. This view lost its appeal, however, when it became evident that the Curie-temperature is exceeded in the Earth below a depth of about 30 km. Ferromagnetic materials in the Earth's crust could thus explain only magnetic fields with a relatively short wavelength. The current period of intense research on the magnetism of planets other than that of the Earth started with the first detailed measurement of Jupiter's magnetic field by the Pioneer 10 space probe in 1973 and the discovery of Mercury's magnetism by Mariner 10 in 1974. In the early seventies also the development of the theory of magnetohydrodynamic dynamos had started in which the reaction of the Lorentz force of the generated magnetic field is taken into account in physically real-Until that time dynamo theoreti-cians had focused their attention on the kinematic problem in which the possibility of growing magnetic fields driven by somewhat arbitrarily chosen velocity fields is considered. It must be remembered that only a few years earlier it had been demonstrated by Backus (1958) and Herzenberg (1958) in a mathematically convincing way that the homogeneous dynamo process of the generation of magnetic fields in a singly …