Ducted and partly ducted propagation of VLF waves through the magnetosphere

Abstract

The ducted and partly ducted propagation of artificial VLF waves through the magnetosphere has been studied by means of data from the FR-1 satellite experiment, in which fixed-frequency (16.8 kHz) waves radiated by a transmitter on the ground (at L = 2.1) are received by the satellite at 750 km altitude both in the zone close to the transmitter and in the conjugate zone. From the wave-normal directions and Doppler shifts measured in the conjugate zone, it has been possible to identify waves that have been ducted along almost the whole of their magnetospheric path (in the case where the wave frequency is less than half the electron gyrofrequeney everywhere on the path) or along only a part of their path (in the case where the wave frequency is equal to half the electron gyrofrequeney somewhere on the path). It is shown that generally the lower ends of the ducts are situated above the altitude of the satellite, but that on some occasions the ducts extend down into the topside ionosphere and so can be observed at this altitude. Ducted propagation is observed to be a quasi-permanent feature of the VLF field between L = 1.7 and 3. Below L = 1.7, only non-ducted propagation is observed.