Scattering of ELF radio waves by a localized non-uniformity in the lower ionosphere

Abstract

We numerically model the ELF radio wave scattering by a localized non-uniformity in the mesosphere conductivity characterized by a 25–30 km reduction of conventional profile from 70 to 80 km altitude. The disturbance is axially symmetric; it depends on the radius according to the Gaussian law of 1 Mm scale. The complex characteristic electric and magnetic heights are found by solving the Riccati equation. These heights are used in the 2D (two dimensional) telegraph equations (2DTE) when computing the ELF fields in the Earth–ionosphere cavity. The field source is located at the point (10° S; 0° E), the non-uniformity is positioned above the North Pole, and the observer moves across the entire surface of the Earth. Spatial distributions were computed of the vertical electric field amplitude in both regular and non-uniform cavity. Comparison of these data allowed us to single out the reflections from the localized non-uniformity. It is shown that interference takes place at an arbitrary observation point between direct wave and the wave reflected from the non-uniformity. Computational data might be interpreted by secondary currents induced in the non-uniformity by the incident radio wave. There are two types of secondary sources. The first one is a “monopole” radiating symmetrically in all directions. Spatial distribution of the field from this source coincides with the distribution of corresponding Schumann resonance mode at a particular frequency. The other secondary source is of “dipole” type, which has a cosine radiation pattern with the maximum oriented along the source – non-uniformity line. Its spatial distribution coincides with the derivative of the corresponding Schumann resonance mode eigen-function on the distance. Model data obtained will facilitate interpretation of experimentally observed seismogenic Schumann resonance signals.