Heating of thermal helium in the equatorial magnetosphere: A simulation study

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Abstract
Heating of heavy ions is observed in the equatorial magnetosphere in conjunction with ion cyclotron waves generated by anisotropic hot protons (i.e., GEOS 1 and 2 and ATS 6 results). The mechanism of the heating is studied by a numerical simulation. The plasma parameters which have been chosen are those which prevail in the dayside magnetosphere at geostationary altitudes. The plasma consists of cold and isotropic H+ and He+ ions (κT/2 ∼ 1.7 eV) with a small number of hot anisotropic protons which provide the free energy necessary to generate the waves. The code is one‐dimensional in length and three‐dimensional in velocity. It is electromagnetic and hybrid; i.e., the electrons are treated as a massless fluid. The results obtained during the linear phase (t ≤ 150ΩH−1) are in agreement with those expected from the linear theory as far as the growth rate of the wave (and the frequency of the most amplified wave) and the variation of the hot proton anisotropy are concerned. The saturation (Bwave ∼ 0.05Bo) is explained by trapping of the helium particles. But the most interesting results concern the heating of cold species. He+ ions are heated mainly in the perpendicular direction (kT⊥/2 ≃ 150 eV, kT∥/2 ≃ 80 eV), and they are heated more than cold H+ ions (kT⊥/2 ≃ 15 eV, kT∥/2 ≃ 20 eV). The heating of He+ ions is a two‐step process: first, He+ ions are set into oscillations by the growing wave (in both υ⊥ and υ∥) until some of the ions reach a parallel velocity of the order of the resonant velocity, at which time strong heating occurs. Phase‐space plots for the different particle species at different times illustrate the time evolution of this heating mechanism.