An experimental study of the proton whistler, a new VLF phenomenon observed in satellite data, is presented, and an explanation of this new effect is given. The proton whistler appears on a frequency-time spectrogram as a tone which starts immediately after the reception of a short fractional-hop whistler at the satellite and initially shows a rapid rise in frequency, asymptotically approaching the gyrofrequency for protons in the plasma surrounding the satellite. It is proposed that the proton whistler is simply a dispersed form of the original lightning impulse and that the dispersion can be explained by considering the effect of ions on the propagation of an electromagnetic wave in the ionosphere. The propagation of a wave in a multicomponent plasma for frequencies of the order of the ion gyrofrequencies is discussed. In the ionosphere it is found that, in addition to the right-hand polarized whistler mode, the left-hand polarized mode (ion cyclotron wave) is also a possible mode of propagation for certain ranges of frequencies and altitudes. Between each two adjacent ion gyrofrequencies there is a frequency for which both modes of propagation are linearly polarized. These frequencies are called the crossover frequencies. A wave propagating in the ionosphere changes polarization at the altitude where the wave frequency is equal to a crossover frequency. This polarization reversal provides the mechanism by which an upgoing whistler can become an ion cyclotron wave. We show that the proton whistler is an ion cyclotron wave which occurs via this polarization reversal process. The crossover frequency can be measured from spectrograms of proton whistlers and is used to determine the fractional concentration of H+ in the plasma surrounding the satellite. Near the altitude and frequencies for which polarization reversal occurs, it is shown that the right-hand polarized wave and the ion cyclotron wave may be strongly coupled. For frequencies of the order of the ion gyrofrequencies, this coupling process plays an important part in determining what regions of the ionosphere are accessible to waves from a given source location.
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