Exos-B/Siple Station VLF wave-particle interaction experiments 2. Transmitter signals and associated emissions

Interactions between coherent VLF waves and energetic particles in the magnetosphere have been studied in a joint program involving the Japanese high-altitude satellite EXOS-B and the Siple Station VLF transmitter. During the period July 15--September 7, 1979, transmissions to the EXOS-B satellite were carried out on 50 separate occasions when the spacecraft was within +- 600 longitude of the magnetic field lines linking Siple Station, Antarctica (760S, 840W geographic, Lapprox.4.1) with its conjugate station at Roberval, Canada. Of this total, 37 were carried out while the satellite was located in the 1000--1600 LT sector of the magnetosphere and 13 while in the 0300--0800 LT sector. The transmitter signals were detected on EXOS-B on 50% of the occasions when transmissions were attempted, and on 5 occasions the transmitter signals were observed to have triggered VLF emissions somewhere along their ray path between the ionosphere and the satellite. All 5 triggering events occurred in a 6-day period following a large magnetic storm that took place on August 13, 1979, with 2 events occurring in the 1130--1330 LT sector and 3 events occurring in the 0400--0600 LT sector. Analysis of the emission triggering events provided strong evidence that the triggering took place inside whistler-mode ducts and that the emissions reached the satellite only after being scattered at one end of the ducts by ionospheric irregularities. On at least one day, emissions were triggered by ducted echoes of the transmitter signals but not by the direct ducted pulses themselves. On another day a new type of triggered emission was observed. We conclude that in the noon sector of the magnetosphere the amplitude of nonducted signals from the Siple transmitter is generally less than the threshold level necessary for triggering in the nonducted mode