THE LOWER CHARACTERISTIC ELF ALTITUDE RESONANCE OBSERVATIONS AND AERONOMICAL ESTIMATES OF THE EARTH-IONOSPHERE WAVEGUIDE: SCHUMANN

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: theoreti- cally, 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 Schu resonance (SR) frequency range. A simple tech- nique 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 fre- quency range (SR I to III).