The lower characteristic ELF altitude of the Earth-ionosphere waveguide: Schumann resonance observations and aeronomical estimates

Propagation of ELF fields in the Earth-ionosphere waveguide is treated in terms of a two-dimensional method with two complex characteristic altitudes as actual propagation parameters. The day-to-night variations of the lower characteristic altitude are estimated in two different ways: theoretically, on the basis of the parameter's physical model and representative day- and night-time conductivity profiles for the lower ionospheric D-region, and experimentally, from systematic observations of the background electromagnetic signal in the Schumann resonance (SR) frequency range. A simple technique for considerably eliminating undetectable factors (the average current moment, the ionospheric properties over sources, etc.) and smoothing resonance effects in experimental data is applied. It is found that theoretical-aeronomical and experimental estimates are in a reasonable agreement, showing an approx. 20 to 25% day-to-night increase in the lower characteristic altitude within the lower SR frequency range (SR I to III).