Two-dimensional hybrid code simulation of electromagnetic ion cyclotron waves of multi-ion plasmas in a dipole magnetic field

A two-dimensional hybrid code (particle ions and fluid electrons) is used to simulate EMIC waves in a H+-He+-O+ plasma in a dipole magnetic field. The waves are driven by energetic ring current protons with anisotropic temperature (T-perpendicular to p/T-parallel to p > 1). The initial state of the plasma is derived from an anisotropic MHD code so that the system is in MHD equilibrium, J x B-del.P = 0. The cold species (with temperature of similar to eV) are assumed to be isotropic and have a spatially uniform density distribution. We choose our parameters so that the EMIC waves are generated near the magnetic equator with frequencies Omega(O)+ < omega < Omega(He+). The presence of each heavy-ion species introduces a new dispersion surface. When the waves grow near the equator, they are dominantly left-handed polarized and have small wave normal angle. While propagating toward high latitudes, the waves become linearly or right-handed polarized with a larger normal angle, and they encounter the second harmonic of the O+ cyclotron frequency, the He+-O+ bi-ion frequency, and possibly the first harmonic of the O+ cyclotron frequency. In this process, some waves are absorbed by the wave-particle interaction, some waves are reflected by the He+-O+ bi-ion frequency, some are transmitted on the same dispersion surface, and some may tunnel through the so-called stop band. The relative importance of these effects varies with the ion composition and especially with the concentration of O+, eta(O+) = n(O+)/n(e). For instance, for eta(O+) << 0.5%, essentially all the wave energy passes through the resonances to reach the ionospheric boundary. For eta(O+) = 0.5% (the case examined in most detail), the time-averaged Poynting vector at high latitudes is almost always in the poleward direction, even though clear evidence of some reflection at the He+-O+ bi-ion resonance is seen.