A controlled VLF phase-reversal experiment in the magnetosphere

Abstract : During the 1973 operations of the transportable very-low-frequency (TVLF) transmitter near Anchorage, Alaska (L approx. 4), an experiment was performed to determine the effect of controlled phase changes of the transmitted wave on the magnetospherically propagated signal received in the conjugate region. At periodic intervals, the phase of the driving voltage was changed (essentially instantaneously) by 180 deg. The amplitude of the 6.6 kHz signal detected in the conjugate region went to zero and recovered with a characteristic time constant of 33 msec. This is ten times longer than the antenna current response time and is in fact comparable with characteristic electron interaction times with whistler-mode waves. Between the times at which the phase reversals occurred, the received signal was amplitude modulated. The period of the modulation was about 26 msec. An upper sideband was present in the spectrum while these pulsations were occurring. These characteristic times are in general agreement with theoretical predictions of bandwidths, growth rates, and particle trapping frequencies for whistler instabilities in the magnetosphere. Data obtained from the controlled transmissions and from lightning generated whistlers propagating in the same duct were combined to determine the plasma and wave parameters at the geomagnetic equator. Of particular interest is the level at which the magnetic field of the wave saturated. During the time period for which the data were analyzed, this was found to be 3.5 pT. (Author)