Hydromagnetic oscillations of the Earth's core

The dispersion relationship for hydromagnetic waves in a rotating system indicates that, under plausible assumptions concerning the strength of the toroidal magnetic field in the earth's liquid core, the periods associated with slow planetary‐scale waves in the core would be comparable with the characteristic time scales found in the geomagnetic secular variation, decades to centuries, and it now seems likely that such waves could also play a key role in the production of the geomagnetic field by the ‘dynamo’ process. Attempts to interpret specific features of the field in terms of hydromagnetic oscillations of the core (or of any other type of fluid motion, for that matter) have not yet proved entirely successful, for they encounter serious and in some cases fundamental mathematical difficulties. The present paper reviews theoretical work on free hydromagnetic oscillations of a bounded rotating fluid in the context of the geomagnetic problem.