The recent spacecraft observations by MMS and Van Allen Probes associated with electromagnetic ion cyclotron waves (EMIC) in the Earth magnetosphere emphasize the important role of multi-ion plasma composition for generation and characteristics of these emissions. We show that the main properties of EMIC waves can be explained with the concept of ‘multi-ion oscillitons’ (Sauer et al., 2001). In a plasma with two types of ions of different masses (e.g. protons and oxygen ions), oscillitons arise from the exchange of momentum and energy between the two ion components, with the electromagnetic field acting as a mediator. At frequencies near cross-over frequencies of different wave modes in the multi-ion plasma the nonlinear resonance which strongly amplifies the seed unstable mode can be excited. A small phase difference in oscillations of different ion species leads to a nonlinear wave beating and generation of wave packets. The ‘resonance ‘ frequency is characterized by a local maximum of the phase velocity and the coincidence of phase and group velocity. The generation of coherent waves by oscillitons is of a general nature and may contribute to understand the manifold of phenomena in other space plasma environments in which the dynamics of minor ion admixtures cannot be neglected, as alpha particles in the solar wind and heavy ions around active comets. The concept of oscillitons also applies to the momentum exchange between different particle groups of the same mass. On this way, whistler oscillitons may arise in two-temperature electron plasmas.

The recent spacecraft observations by MMS and Van Allen Probes associated with electromagnetic ion cyclotron waves (EMIC) in the Earth magnetosphere emphasize the important role of multi-ion plasma composition for generation and characteristics of these emissions. We show that main properties of the coherent EMIC waves can be explained with the concept of ‘multi-ion oscillitons’ (Sauer et al., 2001). In a plasma with two types of ions of different masses (e.g. protons and oxygen ions), oscillitons arise from the exchange of momentum and energy between the two ion components, with the electromagnetic field acting as a mediator. At frequencies near cross-over frequencies of different wave modes in the multi-ion plasma the nonlinear resonance which strongly amplifies the seed unstable mode can be excited. A small phase difference in oscillations of different ion species leads to a nonlinear wave beating and generation of wave packets. The ‘resonance ‘ frequency is characterized by a local maximum of the phase velocity and the coincidence of phase and group velocity. It is suggested that the oscillitons are triggered by the instability due to the proton temperature anisotropy and may propagate outside the source region. The generation of coherent waves by oscillitons is of general nature and may contribute to understand the manifold of phenomena in other space plasma environments in which the dynamics of minor ion admixtures cannot be neglected. The concept of oscillitons can also be applied to the momentum exchange between particle groups of the same mass, but different temperature.

In this paper, the modifications of the whistler dispersion characteristics are investigated which arise if resonant electrons are taken into account. The following chain of processes is emphasized: Generation of whistler waves propagating at different angles to the magnetic field and their nonlinear interaction with resonant electrons result in the appearance of modulated electron beams in the background plasma. As a result, the dispersion characteristics of waves in this new plasma might be significantly changed. By solving the kinetic dispersion relation of whistler waves in electron plasmas with so-called beam/plateau (b/p) populations, the associated modifications of the whistler dispersion characteristics are presented in diagrams showing, in particular, the frequency versus propagation angle dependence of the excited waves. It is important to point out the two functions of the b/p populations. The interaction of the beam-shifted cyclotron mode ω = Ωe + k·Vb (Vb<0, Vb is the b/p velocity, Ωe: electron cyclotron frequency)) with the whistler mode leads to enhanced damping at the ω-k point where they intersect. This is the origin of the frequency gap at half the electron cyclotron frequency (ω~Ωe/2) for quasi-parallel waves which are driven by temperature anisotropy. Furthermore, it is shown that the upstream b/p electrons alone (in the absence of temperature anisotropy) can excite (very) oblique whistler waves near the resonance cone. The governing instability results from the interaction of the beam/plateau mode ω=k·Vb (Vb>0) with the whistler mode. Relations to recent and former space observations are discussed.