Complex demodulation and electromagnetic response function for geomagnetic response function for geomagnetic field variations at 27-day and its harmonics

Abstract

To determine the upper mantle electrical conductivity distribution beyond the depths probed by Sq-periods, the daily mean geomagnetic field variations for the years 1975-1977, recorded through Indo-Russian chain of observatories is subjected to complex demodulation technique to isolate the space-time evolution of periodic field variations for 27-day and its harmonics. The data from this unique latitudinal distribution of 13 observatories has facilitated to best utilize and test statistically the implicit assumption on the representation of spatial characteristic of the field by spherical harmonic term PI0 (cos θ). The isolated demodulates which satisfied the P10 dependence are used to obtain the depth and conductivity of the substitute conductor for period bands of 25-32, 11-16 and 8-10 days, representative of 27-day and its harmonics. These results when combined with those obtained using Sq-periods help to visualize the mantle in the depth range of 50-1200 Km. as a stack of inhomogeneous layers with a clear suggestion of two layers of moderately high conductivity at 125 and 275 Km. and a sharp discontinuity in the depth range of 350-500 Km. The electrical conductivity and the depth of the conductosphere for the period band of 25-32 days are estimated to be 0.33 S/m and 1200 km. respectively. Both these estimates agree quite well with the values reported for southern Chinese and European regions, suggesting a laterally homogeneous electrical structure of the mantle at such great depths.