The Magnetic Field and the Central Core of the Earth*

The suggestion that the inner core of the Earth is liquid and is the seat of a strong magnetic field is examined. Contrary to expectations of a magneto-hydrodynamic rigidity for the inner core, it is shown that a liquid inner core with a strong magnetic field should not transmit shear waves. The absence of the phase PKJKP in seismic records is favourable to the hypothesis of a strong magnetic field in a liquid inner core. If the jump in P wave velocity at the inner core boundary is due to a magnetic field, the strength of the field must be of the order of 5 x 106 G whether the field geometry is toroidal or random. A strong toroidal field leads to an apparent ellipticity of the inner core. No such ellipticity is observed, so that if the field in the inner core is toroidal, it must be less than 5 x 105 G. A random field does not lead to an ellipticity effect and no limit can be set on the magnitude of a random field. The energy required for a magneto-hydrodynamic rigidity in the inner core is excessive. The magnetic energy is equally divided between the inner and outer cores. The hypothesis of a strong magnetic field in the inner core cannot explain the observed short period variations of the surface field. I