Controlled stimulation of VLF emissions from Siple Station, Antarctica

Coherent VLF (1.8–7 kHz) signals injected into the magnetosphere from Siple Station (L = 4.2), Antarctica, are amplified (30 dB) and trigger intense, narrowband whistler mode emissions. The mechanism of growth is believed to be wave‐induced phase bunching of cyclotron resonant electrons in an interaction region near the equatorial region. A key parameter in these experiments is the spectral purity of the transmitted signal. If the width of the signal spectrum exceeds ∼ 10 Hz, there is significant loss of gain. In one experiment ( f ≈ 3 kHz), two equal‐amplitude signals are transmitted with a frequency spectrum of Δ f . When Δ f = 20 Hz, the gain is reduced as much as 20 dB below its single‐frequency value. For f ≥ 100 Hz, the two signals tend to behave independently of one another, indicating that they interact with different electron populations. Other effects of interest include entrainment of strong free‐running emissions by a relatively weak Siple signal and the frequent generation of sidebands (2–100 Hz) on Siple signals that have reached saturation. Future work at Siple Station will emphasize frequencies below 4 kHz in order to raise the energies (to > 10 keV) of the precipitating electrons and hence make them more readily detectable by ground‐based techniques. To achieve this goal, the dipole length will be doubled (to 43 km), which is expected to increase the maximum radiated power at 2.5 kHz by 7 dB. Later, it is planned to install a circularly polarized crossed dipole that will double the effective radiated power and permit new experiments on the polarization properties of the ionosphere and magnetosphere at VLF.