Upper mantle conductivity determined from the solar quiet day ionospheric currents in the dip equatorial latitudes of West Africa

Abstract

The magnetic data obtained from a chain of ten magnetotelluric stations installed in the African sector during the international equatorial electrojet year (IEEY) was used to establish the 1993 quiet day current system (Sq) for West Africa and to determine the Earth’s upper mantle electrical conductivity in the region. A spherical harmonic analysis (SHA) was applied in the separation of the internal and external field/current contribution to the Sq variations, while a special transfer function was used to compute the conductivity – depth values from the paired external and internal coefficient of the SHA. The variation in the currents is seen to be a dawn to dusk phenomenon with the variation in the external currents different from that of the internal currents both in amplitude and in phase. The seasonal variation in the external current maximizes during the March equinox and minimizes in the December solstice. The conductivity had a downward increase with a high conductivity region spotted between 100 km and 205 km, which is seen to correspond to the seismic low velocity region. The conductivity at the upper mantle is seen to be 1.05 times higher than that obtained both in the Asian (Himalayan) and Australian regions.