Multi‐Ion Oscillitons—Origin of Coherent Magnetospheric EMIC Waves

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, https://doi.org/10.1029/2001GL013047). 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. Plain Language Summary The mode splitting of electromagnetic waves at oblique propagation in plasmas with multiple ion species leads to the creation of gap regions in omega‐k space. In these “forbidden regions” spatially growing waves exist whose nonlinear state represents a new type of solitons. These so‐called oscillitons, first described by Sauer et al. (2001, https://doi.org/10.1029/2001GL013047), arise from momentum and energy exchange between two or more ion components, with the electromagnetic field acting as a mediator. We suggest that multi‐ion oscillitons are the origin of the ion cyclotron electromagnetic waves (EMIC) in the magnetosphere, which have been known for a long time. Valuable insights have recently been gained through improved diagnostics on the satellites MMS and Van Allen Probes. Key Points Mode coupl