Discrete ELF/VLF signals, i.e. those whose strength exhibits a well-defined signature in the frequency–time domain (see Fig. 1.3), abound in the Earth's magnetosphere. Whistlers generated by lightning discharges give important information on the distribution of plasma above the ionosphere. They are an effective diagnostics tool for near-Earth plasmas, and their study led to the discovery of the plasmapause and the plasmasphere. We shall be interested here in other types of discrete ELF/VLF signals, which include artificial signals termed triggered emissions excited by external quasi-monochromatic waves from ground-based transmitters, and so-called chorus radiation. Both of these discrete emissions are generated by gyroresonant interactions of whistler-mode waves with radiation belt electrons, and are described here on the base of the nonlinear theory presented in Chapters 3–7.

In this chapter we mainly consider quantitative descriptions of the generation of these signals, referring for details of the history and for other theoretical details to the above mentioned chapters and to the reviews by Helliwell (1969, 1988, 1993), Matsumoto (1979), Omura et al. (1991), Trakhtengerts and Rycroft (2000), and Demekhov and Trakhtengerts (2001).

Overview of experimental data on triggered VLF emissions

The typical frequency of a triggered VLF emission is below half the electron gyrofrequency in the equatorial plane of the magnetic flux tube where the emission is generated. The triggering signals are pulses of length from 0.05 to 10s produced by terrestrial VLF (3–30 kHz) radio transmitters with bandwidths normally of the order of, or less than, 20 Hz.