Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.137..630k&link_type=abstract
Geophysical Journal International, Volume 137, Issue 3, pp. 630-650.
Astronomy and Astrophysics
Astronomy
15
Geomagnetic Variations, Leakage Currents, Numerical Models, Oceans, Spherical Shell Models, Transverse Resistivity
Scientific paper
We pursue in this paper the following goals. (1) Detailed study of the ocean effect on geomagnetic fields excited by Sq and Dst sources in spherical earth models. The investigated period range is from 6 hr to 10 days. Oceans and continents form a thin shell of variable conductance above a layered deep structure, allowing for leakage currents from and into the shell. (2) Development of an interpretation scheme for the estimation of the resistivity rho_l of the lithosphere. (3) Application of this scheme to estimate rho_l from a comparison of calculated and observed Sq harmonics at coastal sites. We conduct a number of model calculations with simplified hemispherical shells and with realistic shells for the world's oceans and continents, deriving their variable conductance from the ocean depth at sea and assuming a uniform value of 400 S on land, with inclusion of continental shelfs. From these model studies we draw the following conclusions. (1) Variations of rho_l in the uppermost 100 km of the deep structure significantly change the vertical magnetic surface field Z near oceans. (2) Ocean effects in the horizontal magnetic surface fields X and Y are quite negligible on land for Sq. Therefore, radially symmetric earth models can be used to determine the external Sq source field from a spherical harmonic analysis of X and Y on land. (3) With realistic shell models it is possible to reproduce the anomalous coast effect in Z of Sq. A two-step interpretation scheme is presented to estimate the resistivity rho_l of the lithosphere. In step 1 we determine the spherical harmonic coefficients of the external Sq source field using the observed Sq variations in X and Y and a presumed earth model of uniform shell conductance to separate their external and internal parts. In step 2 we derive model fields with various trial values of rho_l for a realistic surface shell excited by the external source field from step 1. We choose the most likely value of rho_l, that for which the rms misfit between observed and calculated Z variations is smallest. Our observational data are the first four Sq harmonics of the four equinoctial months of 1964 as derived from the mean daily variations of quiet days at 76 observatories. Z observations at 12 coastal observatories lead us to the conclusion that a global estimate of lithospheric resistivity rho_l is of the order of 10 000 Omega m.
Avdeev Dmitry B.
Kuvshinov Alexei V.
Pankratov Oleg V.
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