Magnetotelluric sounding and electromagnetic diagnostics of the magnetosphere and lithosphere: Possibilities of complementarity

Observations of ultra-low-frequency (ULF) variations in geomagnetic and electrotelluric fields at a network of ground stations are the experimental basis for several geophysical areas: magnetotelluric sounding (MTS) of the earth’s crust; hydromagnetic diagnostics of the near-Earth plasma; electromagnetic monitoring of dynamic processes in the lithosphere. The proposed review will demonstrate the possibilities of mutual influence of these seemingly dissimilar areas. Magnetospheric resonance effects can cause distortions of the MTS curve over low-conductive layers near the local resonance frequency, which can be misinterpreted as a feature of the earth’s crust structure. On the other hand, a new method of hydromagnetic diagnostics may be adopted that uses both magnetic and electric components of ULF field variations, which can determine the latitudinal variation of the resonance frequency based on data from one observation site. When searching for electromagnetic precursors of earthquakes, we can take an opportunity to separate magnetospheric and seismogenic disturbances, relying on the fact that for ionospheric sources the apparent impedance coincides with the surface impedance of the earth, but the impedance of disturbances created by a lithospheric source exceeds it by an order of magnitude. The question about the presence of an electric mode in the field of geomagnetic pulsations incident on the earth's surface, which is important for MTS sounding, is still controversial. The model of interaction between the Alfvén wave and the ionosphere developed in physics of MHD waves shows a weak excitation of the electric mode. The generation of artificial ULF signals by power lines as a horizontal radiating mega-antenna makes it possible to conduct MTS over a large area.