The electrical conductivity of the Earth and planets

In addition to a review of global induction studies of the Earth, this paper contains the results of recent investigations of the electrical conductivity of the lunar interior. The profiles obtained by application of the time-dependent transient response technique and the frequency-dependent Fourier harmonic method show some significant differences, particularly in the existence of conductivity 'spikes' in the latter profiles. With this exception, there is agreement that the conductivity increases from about 10-6 S m-1 near the lunar surface to about 10-3 S m-1 at a depth of about 500 km, with the inferred temperature profile increasing from about 00C to about 12000C within this depth range. Information regarding our present knowledge of the magnetic environment of Venus, Mercury, Mars and Jupiter is tabulated. Increasing evidence of lateral variations in electrical conductivity of the Earth's crust and upper mantle has resulted from the numerous regional induction studies undertaken during the past decade using the geomagnetic deep-sounding and magnetotelluric techniques. These are summarized on a regional basis with a fuller discussion of studies undertaken in the UK and in Kenya. The instrumentation used in the observatory, and for field and space observations of the magnetic field and for associated laboratory measurements are described. (author)