Simulations of heavy ion heating by electromagnetic ion cyclotron waves driven by proton temperature anisotropies

Abstract

Heating of heavy ions by the electromagnetic ion cyclotron (EMIC) waves, which are driven by proton temperature anisotropies, is studied by means of hybrid particle simulations. Initially, relaxation of the temperature anisotropies in the proton distribution and isotropic heating of the heavy ions are observed (phase I), followed by substantial perpendicular heating of the heavy ions (phase II). The heavy ions are distinctly gyrophase modulated by the EMIC waves. The isotropic heating in phase I is due to magnetic trapping by the excited proton cyclotron waves. The perpendicular heating in phase II is attributed to cyclotron resonance with the EMIC waves, which becomes possible by means of the preceding heating in phase I. Saturation of the EMIC instability is instead attributed to magnetic trapping of the majority ions: protons. When the proton anisotropy is very large, (T⊥/T∥)p0 ≫ 1, frequency shift (decrease) of the proton cyclotron waves to less than ½Ωp is observed. Even if (T⊥/T∥)p0 ∼ 1, the EMIC waves are excited, and cold helium ions are well heated and gyrophase modulated. The present mechanism is not only relevant to He+ heating in the dayside equator of the magnetosphere, but it also predicts hot He2+ ions behind the earth's bow shock.