Magnetohydrodynamics of the Earth's Core

The problem of explaining magnetic fields of the Earth or the stars is one to which no completely satisfactory solution has yet been given. Permanent magnetism is ruled out because of high temperatures existing in the interior of the bodies. The only theory that appears plausible to explain the existence of magnetic fields is the dynamo theory. For many years, astrophysicists and geophysicists have sought to explain how the motion of an electrically conducting fluid could maintain a steady magnetic field on the earth or a star. The fact that electrical engineers can build self-exciting dynamos is irrelevant because these involve voids, insulators and magnetic materials which are not present in stars. It is now recognized that because diffusion of magnetic field is very small in view of the large size of stars, the fields observed in these bodies are undecayed primeval fields needing no explanation in terms of dynamo action. The earth’s field, however, is widely accepted as being due to dynamo action in its liquid core. This ascribes the maintenance of the magnetic field to electric currents induced in the liquid as a result of its motion across the lines of force, so that there is a general resemblance to the behavior of a self-exciting dynamo.