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A 1-D hybrid numerical code with particle ions and fluid electrons is used to follow the evolution of parallel propagating, low- ($<$ proton cyclotron) frequency, monochromatic wave trains. With warm protons, wave trains undergo instabilities and saturate in a manner which is completely different from pure fluid evolution, due to resonant and nonresonant ion Landau damping. We demonstrate this clearly by showing the evolution of a left-handed wave train with $\beta > 1$ where instability exists for wavenumbers both below and above that of the initial pump wave. For corresponding parameters a fluid theory gives only a narrow range of instability above the pump wavenumber where decay and beat instabilities can occur. In simulations energy spectrally transfers to mostly forward going waves of smaller wavenumber which differs from both decay and beat.
In high speed streams helium ions are an important constituent which travel faster than protons and along the magnetic field. Fluid theory in this case gives new wave train instabilities and an electrostatic beam instability. We present simulations with helium ions having densities and temperatures typical of their observed values. Wave train instabilities remain similar to those without helium. When the beam travels faster than protons at twice the Alfven speed or more, it undergoes an instability which is primarily electromagnetic and greatly slows the beam. This process might regulate its streaming speed in the solar wind.
The implication of this study is that ion kinetics is not a passive factor on the evolution of waves and their spectral transfer. Landau damping, which is often invoked in the solar wind to limit the presence of compressional waves, not only damps these but can also alter the evolution of waves which are primarily electromagnetic. This possibility has not been considered in current solar wind turbulence models or in magnetohydrodynamic calculations of pseudo-sound.
