GOEMAGNETIC VARIATIONS AND ELECTRICAL CONDUCTIVITY OF 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. Copyright © 1969, Wiley Blackwell. All rights reserved