On the suppression of wave growth and triggering in VLF multi-wave experiments

Abstract

In experiments employing two monochromatic whistler-mode wave trains, simultaneously injected from Siple Station, Antarctica, two kinds of suppression phenomena were identified and recently reported by Helliwell (1983). The first occurs for small frequency separations (∼ 20–30 Hz) and involves the mutual suppression of wave growth and triggering of both signals; the second type of suppression is unsymmetrical: it occurs only in the lower frequency wave, for frequency separations larger than those for which mutual suppression is detected. Here, we discuss the mechanisms underlying these effects based on a wave-particle interaction description, in the presence of two waves. Mutual suppression of wave growth and triggering is understood as a consequence of the mutual inhibition of the waves to phase trap resonant electrons: this is expected to perturb the development of transverse resonant currents supported by the electrons, thereby causing suppression of growth and triggering of both waves. In what concerns the origin of the assymmetric-type suppression occurring in the lower frequency ( ƒ 1 ) wave, a different two-wave interaction mechanism is contemplated: we assume the suppression to be related to the perturbing effects caused by the higher frequency ( ƒ 2 ) wave on the electron population that would have resonated with ƒ 1 in the absence of ƒ 2 . These effects shall depend on the capability of ƒ 2 to trap these electrons, thus avoiding their resonant interaction with ƒ 1 and consequently affecting its growth. We analysed, theoretically and by computer simulation, the influence of the perturbing second wave on the energetic electrons that would have resonated with the first wave. A model was developed to quantify the critical frequency spacing associated with unsymmetrical interaction effects and a comparison was made with the critical frequency spacing associated with mutual interaction effects. For the considered range of frequencies, corresponding to whistler-mode ducted propagation in the geomagnetosphere near L = 4, it was shown that the unsymmetrical effect may dominate over the mutual effect, above critical amplitude levels within the moderate range of wave amplitudes (∼ 1–10 pT). A good agreement was obtained between predicted results for critical frequency separations and those reported in the examples corresponding to experiments in the range 3–4 kHz (Heliwell, 1983), where detected unsymmetrical effects seem to extend to frequency separations larger than 200 Hz.