Propagation of extremely low-frequency (ELF) atmospherics over a mixed day-night path

Abstract An experiment was conducted to evaluate the averaging of propagation parameters of extremely low-frequency (ELF) radio waves propagating over a path in which the ionospheric environment changed within one wavelength. The Fourier spectral components of ‘slow-tail’ atmospherics recorded simultaneously at California and Hawaii were used to determine wave attenuation rates and phase velocities for the frequency band 48–290 Hz when the propagation path was either full-day or full-night. A third receiving station in Oklahoma, located near the great circle common to the two remote stations, aided in locating the lightning sources. From measured changes as the night-day transition region traversed the propagation path the averaging of phase velocity and propagation loss was evaluated with respect to percentage of day and night paths from the sources to the receiving stations. Results indicate that a simple weighted average of the day and night phase velocities is a valid description of measured phase velocities across the day-night transition. Less agreement was obtained between the measured propagation loss across the transition region and that predicted from a simple day-night weighted average of wave attenuation rates.