Energization of helium ions by proton‐induced hydromagnetic waves

Authors:
  • RGRoger Gendrin
    Roger Gendrin
  • ARAlain Roux
    Alain Roux
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Abstract
We consider the diffusion of He+ ions under the influence of ion cyclotron waves generated in a plasma consisting of three different ion populations: a thermal isotropic population containing both H+ and He+ ions and an energetic H+ population, with a positive anisotropy A=T⊥/T∥‐1. We compute, in the velocity space υ∥, υ⊥, the diffusion curves that He+ ions will follow in the presence of ion cyclotron waves propagating in such a medium. We show that for small concentrations of the He+ ions, of the order of 1 to ∼10%, these ions can be energized by such a process up to and above suprathermal energies (E ≳20 eV). On some occasions the He+ ions may even reach energies of the order of the Alfvén energy of the cold plasma population: Ea ≈ mp Va²/2 ≈ 5 keV. Characteristic diffusion times, in pitch angle and energy, for both ion species, are evaluated. They are of the order of 2 to 20 min. These theoretical results are discussed in the frame of recent observations by Geos experimenters showing the close association that exists between the occurrence of ion cyclotron ULF waves and the presence of thermal or supra‐thermal He+ ions in the equatorial region of the magnetosphere (Young et al., 1979).

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