Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002geoji.151..689f&link_type=abstract
Geophysical Journal International, Volume 151, Issue 3, pp. 689-709.
Astronomy and Astrophysics
Astronomy
22
Electrical Conductivity, Electromagnetic Induction, Geomagnetic Variation, Ionosphere, Magnetosphere, Statistical Methods
Scientific paper
We report on a new compilation of functions representing the geoelectromagnetic response of the Earth's mantle. The c response functions and newly defined d response functions, the latter of which constitutes a sensitive indicator of lateral heterogeneities in conductivity, were estimated for periods from 5 to 106.7 days. The spatial coverage and statistical quality of these response-function estimates were improved over previous compilations, in part by making use of a newly assembled global catalogue of magnetic observatory hourly mean values, and by applying a statistically robust form of empirical orthogonal function (EOF) analysis. The EOF results suggest that for periods longer than 5 days, the geomagnetic field variation can be represented predominantly by a single eigenmode. We have identified that the primary-source fields of the dominant mode consist of two current systems in the same direction: an equatorial ring current in the magnetosphere, and two conjugate auroral oval ring currents in the ionosphere. The influence of the auroral current system is seen at surprisingly low latitudes, 40° and 50° for the vertical and northern components, respectively, which results in non-negligible bias on the conventional c response at high latitudes. A preliminary correction of the auroral current effect has been made on the c and d responses obtained for data from 55 and 52 observatories, respectively, at latitudes from -60° to 60°. We also present a set of approximate equivalent internal (mantle) electrical current distributions that prove a useful indicator of the presence of deep Earth heterogeneities. These new sets of electromagnetic information will increase the resolving power of 3D distribution of electrical conductivity in the Earth's mantle.
Fujii Ikuko
Schultz Adam
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