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キーワード:
origin of EMIC waves; multi-ion oscillitons; nonlinear waves; Hall-
MHD equations; Earth Science
要旨:
The recent spacecraft observations by MMS and Van Allen Probes associated with electromagnetic ion cyclotron (EMIC) waves 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, <A href="https://doi.org/10.1029/2001GL013047">https://doi.org/10.1029/2001GL013047</A>). 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 survive outside the source region for long distances. 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. The concept of oscillitons can also be applied to the momentum exchange between particle groups of the same mass, but different temperature.
