Magnetohydrodynamic Waves Within the Earth
The fluid core of the Earth is electrically conducting and it is permeated by a magnetic field that makes the propagation of waves, generally called magnetohydrodynamic (MHD) waves, possible. These waves were predicted in the 1940s by Hannes Alfvén, who proved theoretically that the magnetic field lines in a perfectly conducting fluid behave like elastic strings frozen to the medium. It is precisely this elasticity that makes MHD waves possible.
There are several reasons why the study of wave propagation in the core can add insight into the MHD of the core and the origins of geomagnetism. Theoretically, these determine features of core waves that can be put into correspondence with the observations, e.g., the spectrum of core frequencies, the spatial structure of the waves that are most easily excited to large amplitudes, and the rate of drift of the waves about the axis of rotation of the Earth. By choosing the theoretical model whose wave