Geo-electrical structure of the mantle beneath the Indian region derived from the 27-day variation and its harmonics

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Estimates of the C-response function were determined by Z : H method to obtain conductivity and depth values for a substitute perfect conductor (conductosphere) beneath the Indian region, utilizing geomagnetic variations at periods of 27-day and its harmonics. Two and half years of continuous geomagnetic data were utilized. These data were recorded during 1975-77, at a chain of 13 stations confined to the 150⁰ geomagnetic longitude band, which extended from the dip-equator at the southern tip of India, to the northern parts of Russia. Complex demodulation techniquewas employed to determine the electromagnetic (EM) responses. Taking advantage of the dense latitudinal distribution of the observatories, the demodulates of all the stations were tested statistically to check the validity of the P₁⁰ approximation for the inducing field. Single-station response estimates, for a 27-day period, computed by a robust method have shown that reliable EM responses (consistent with P₁⁰ source dependence and with local 1-D Earth structure) could be obtained for only 6 stations, all situated in the mid-latitude region. The depth estimates at all 6 stations are consistent, including Sabhawala (SAB) which is situated close to the Himalayan collision zone. The negligible differences in the depth estimates of these mid-latitude stations do not show any latitudinal dependence, as against such an observation reported for the European and the North American regions. The mean depth of the conductosphere is found to be 1200 (±200) km, with an average conductivity of 0.7 (±0.3) S/m. Comparison of the mean geo-electrical structure with those of other regional models shows that the presence of a mid-mantle conductor at 850 km depth could be considered to be a global phenomenon.