Abstract:
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.
