Electromagnetic ULF Fields from an Underground Seismic Source on the Earth Surface and in the Ionosphere

Abstract—A theoretical formalism has been developed to calculate the electromagnetic fields generated in the atmosphere–ionosphere system by a finite-length underground horizontal current source. A numerical model with a realistic profile of the ionosphere in a vertical geomagnetic field has been constructed based on this theory. It is shown that the apparent impedance of the electromagnetic field produced by an underground source on the Earth surface is an order of magnitude higher than the Earth impedance; this can be used to discriminate perturbations from seismogenic sources. The presented results of numerical modeling allow us to relate perturbations created by a large-scale underground source in the magnetic field of the Earth surface and in the electric field in the ionosphere. Based on these model estimates, it is concluded that many of the ULF electric field perturbations detected in satellite data prior to earthquakes cannot be attributed to direct emission from seismogenic sources.