Propagation of the 50-Hz Spectral Component of ELF Atmospherics Across the Sunrise Terminator

An experimental evaluation of the averaging of attenuation by elf waves propagating over a path in which the environment changes within 1 wavelength. Vertical and horizontal electric-field components of elf atmospherics were simultaneously recorded at two widely separated stations, one in Hawaii and one in Arizona, as the sunrise terminator traversed the propagation paths between a quasi-stationary source of atmospherics and the two receiving stations. From the measured changes in attenuation rate as the day-night transition region traversed the propagation paths, propagation loss was evaluated with respect to the percentages of day and night paths from the sources to the receiving stations. A simple weighted average of the day and night propagation path losses is found to be a valid description of the propagation losses across horizontal inhomogeneities which occur in distances small compared to a wavelength. A directional effect is found to exist on wave attenuation rate for daytime as well as nighttime, and it is indicated that the difference between day and night attenuations may be much greater than previously believed. (Author)