Theory and computer simulations of magnetospheric very low frequency emissions

Abstract

A theory of magnetospheric VLF emissions must account for the following features: (a) the triggering of monochromatic emissions by signals of sufficient strength and duration, while the background noise and weak short signals are not amplified, and (b) the occurrence of frequency changes after the emissions have reached a sufficiently large amplitude. A nonlinear mechanism exhibiting these features, with fixed and varying frequencies, is examined analytically and by computer simulation techniques. This mechanism depends on a simultaneous propagation and amplification of wave packets along geomagnetic lines to maintain the nonuniformity ratio R ∝ ▽B0/Bw in the regime |R| ≈ 0.5, corresponding to maximum amplitication. (B0 is the geomagnetic field and Bw is the wave magnetic field.) For a constant frequency, this condition yields triggering thresholds which are related to the properties of the magnetosphere. For a varying frequency ω(t), it yields the condition ∂ω/∂t ∝ ωt² for the large‐amplitude portion of the risers, where ωt ∝ Bw1/2 denotes the trapping frequency of the wave.

Oblique Whistler-Mode Waves in the Inhomogeneous Magnetospheric Plasma: Resonant Interactions with Energetic Charged Particles

“…As was mentioned above, most theoretical work dealing with the phenomena discussed (see, e.g. AshourAbdalla 1970;Nunn 1971;Palmadesso and Schmidt 1971;Sudan and Ott 1971;Nunn 1974;Karpman et al 1974a, b;Karpman and Shklyar 1977;Vomvoridis et al 1982;Omura and Matsumoto 1982;Inan et al 1982;Bell 1984;Shklyar et al 1992, and references therein) was limited to the case of whistler-mode wave propagation along the ambient magnetic field. The theory presented here permits us to remove this limitation and to enrich theoretical studies of various phenomena related to wave-particle interactions in the magnetosphere.…”

Quasineutral particle simulation technique for whistlers

“…the review by Helliwell [9]), and considerable effort has been devoted to theoretical and numerical studies (see, e.g. [6,[10][11][12][13]), but computational efficiency has been a limiting factor. It is thought that finite wavepacket length, and spatial variation of both the plasma density and the ambient field, play an essential role in the nonlinear evolution of resonant electron/whistler interactions [6,10,11].…”

Beam-plasma instability in inhomogeneous magnetic field and second order cyclotron resonance effects

“…5,6 Such a situation takes place in the Earth's magnetosphere in the case of the socalled triggered ELF/VLF emissions. [7][8][9][10] The beams with a step-like distribution function can appear in space plasma conditions even more often than ␦-functions.…”