Ducted magnetospheric propagation of signals from the Siple, Antarctica, VLF transmitter

Abstract

A study has been made of the conditions under which ≃2- to 6-kHz VLF signals transmitted from Siple, Antarctica (L ≃ 4), are observed at the conjugate ground station Roberval, Canada, following ducted propagation through the magnetosphere. In 1973 and 1974, signals detected from visual inspection of spectrographic records were present on roughly 20% of the days, typically for ≃4 of 8 observing hours. As had been expected, the observations appeared sensitive to conditions of whistler propagation and of wave growth in the magnetosphere. Signal observations were most frequent during quieting following magnetic disturbance. A diurnal activity peak was found at dawn, and day side activity levels were generally higher than those at night. Signal travel times were compared to the dispersion properties of whistlers recorded at Siple and Roberval during July–August 1973 and September–November 1974. The signal path equatorial radii were found to be concentrated in the range 3.5–4.5 RE, which is centered roughly on the Siple/Roberval field lines. At ionospheric heights this range corresponds to a latitude interval of ≃3°. Multipath propagation was frequently observed, but a single path was usually prominent in terms of received amplitude. During relatively quiet conditions the paths were located within the outer plasmasphere; during moderate disturbance they frequently appeared in the region of steep plasmapause density gradients.