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

Abstract

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 150 0 geomagnetic longitude band, which extended from the dip-equator at the southern tip of India, to the northern parts of Russia. Complex demodulation technique was 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 P 0 1 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 P 0 1 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.

On the role of oceans in the geomagnetic induction by Sq along the 210° magnetic meridian region

“…Schmucker (1987) gave full details of the substitute conductor models for different EM response functions in his excellent review. Using the spectral components of geomagnetic Z and H fields, the long period EM induction responses have been calculated to provide the upper mantle electrical conductivity distribution for the European region (Olsen, 1998) and for the Indian and other global regions (Chandrasekhar, 2000 and references therein). Campbell and Anderssen (1983) generalized the Schmucker's (1970) formulation and developed a method to estimate the complex C(ω)-response directly using the spherical harmonic Gauss coefficients for Sq induction studies.…”

External field characterization using CHAMP satellite data for induction studies

“…The estimated global induction response, corresponding depth to the conductosphere and conductivity value are compatible with the earlier observations. For example, using MAGSAT data (Langel 1990;Everett et al 2003;Constable and Constable 2004); using a chain of landbased observatories: for storm-time period data (Chandrasekhar and Arora 1992) and for 27-day period variations and its harmonics Chandrasekhar (2000); using Ørsted satellite data (Olsen 1999); using CHAMP data (Velímský et al 2006). Table 1 shows a comparison between the estimated conductosphere depth and the conductivity value in the present study and the earlier observations.…”

“…The depth to the perfect substitute conductor and its conductivity reported in the present study are in good agreement with the earlier estimated regional and global estimates (table 1). Chandrasekhar (2000) employing the complex demodulation technique and considering only those demodulates, which satisfy the P 0 1 characteristic of the inducing source field, estimated the depth of the conductosphere to be 1200 km with an average conductivity of 0.7 S/m. As observed by Olsen (1998), the EM response obtained by the combined analysis of Sq and geomagnetic storm-time variations for European region also shows a wellmarked conductivity increase at around 800 km depth.…”

“…As a result, several researchers have used a variety of geomagnetic field variations to determine the conductivity and depth estimates of the perfect substitute conductor at regional and global scales. Honkura and Matsushima (1998), Schmucker (1999), Chandrasekhar et al (2003), Kuvshinov et al (2007) and Everett (2010) have used Sq variations; Campbell (1990) and Chandrasekhar and Arora (1992) employed the storm-time variations; Chen and Fung (1991), Olsen (1998), Chen (2000), Chandrasekhar (2000) and Everett et al (2003) have used 27-day variations, its harmonics and beyond, to estimate the depth to the perfect substitute conductor and the conductivity variations at those depth ranges.…”