PROPAGATION CHARACTERISTICS OF WHISTLER-MODE SIGNALS FROM LOW-FREQUENCY DECCA TRANSMITTER

To resolve the propagation characteristics of low-frequency (LF) radio waves in the low latitude magnetosphere, the navigation radio signal (transmitting frequency of 85.725 kHz, 1.2 kW CW) from Biei Decca station in Hokkaido (geographical latitude: 43.60 degrees N, geographical longitude: 142.45 degrees E, magnetic latitude: 37.20 degrees N) was observed at Birdsville, Australia (geographical latitude: 25.83 degrees S, geographical longitude: 139.33 degrees E, magnetic longitude: 37.08 degrees S), which is the magnetic conjugate point. From the analysis of the intensity, Doppler spectrum and polarization of the magnetospheric propagation wave, the following characteristics were obtained. (1) The magnetospheric wave, which is below the detectable level during daytime, appeared around sunset and disappeared immediately after sunrise. (2) at sunset, the width of the Doppler spectrum spreading is rather narrow and the polarization is a right-handed elliptical one. This suggests a propagation in a few ducts. (3) During the night, the width of the Doppler spectrum spreading is rather wide and the polarization is a right-handed elliptical one close to linear. The intensity is higher when the sustaining duration of the magnetospheric propagation wave is longer. This suggests a propagation within several chamber-type ducts, which are repeatedly generated and eliminated within a rather short period. From the calculation of the loss of the ionospheric transmission, difference is suggested in the duct emission mechanism of the magnetospheric duct propagation at the sunset and at night.