AN EXPERIMENTAL INVESTIGATION OF ELF ATTENUATION RATES IN THE EARTH IONOSPHERE DUCT

An instrument system developed for the recording of large amplitude ELF (Extremely Low Frequency) atmospherics or 'events' radiated by lightning in the band 5-50 Hz is described. The attenuation constant of the Earth-ionosphere duct has been deduced from data obtained with the system by modelling the experimentally derived (complex) wave impedance using a numerical optimisation algorithm. This algorithm includes a precise evaluation of the Legendre functions of orders 0 and 1 which define the theoretically predicted fields in the Earth-ionosphere spherical-shell cavity. The data show that the attenuation rate of ELF fields is considerably greater for signals propagating over polar paths than for signals which have propagated on paths confined to tropical and mid-latitude regions. Analysis of the deduced attenuation rate obtained as a function of frequency and arrival azimuth (bearing) has enabled us to estimate the variation of the attenuation rate with wave frequency and latitude, for latitudes greater than that of our observing station at 51-degrees-N. The results provide new empirical information on the variation of the propagation constant of the Earth-ionosphere duct with frequency and latitude.