Geomagnetic Variations and the Electrical Conductivity of the Upper Mantle

The electrical conductivity of the upper mantle can be determined by comparing the measured response of the Earth to magnetic variations of all frequencies with the theoretical response of particular conductivity distributions. On the basis of a limited amount of data the response has been estimated at frequencies in the range 0.003 to 0.25 c day−1. In this range of the geomagnetic spectrum, line spectra at frequencies of 1 and 2 c yr−1 and 1, 2, and 3 cycles per 27 days can be used. Investigations of the continuum spectrum show that it also occurs on a worldwide scale, and must correspond to a real geophysical process. Meaningful estimates of the response can therefore be made over the whole of the frequency range considered. The entire magnetic variation spectrum in the range 2 c yr−1 to 0.25 c day−1 appears to be generated by fluctuations in the strength of the ring current, and a P10 spherical harmonic adequately describes the variation of the magnetic field over the surface of the Earth. The theoretical response of Earth models consisting of concentric shells of variable thickness and conductivity has been computed by putting the boundary problem into a form suitable for the application of matrix methods. The observed P10 response of the Earth is compatible with a distribution in which the conductivity increases sharply by two orders of magnitude at a depth of about 400 km. The width of the region in which this jump occurs is not more than 200 km. It seems likely that the sudden increase in conductivity is caused by a phase transition from the olivine to the spinel form of peridotite. On the basis of this assumption, temperatures have been estimated at depths between 300 and 1000 km.