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

Abstract

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 ( >1). The initial state of the plasma is derived from an anisotropic MHD code so that the system is in MHD equilibrium, J × B−∇·P = 0. The cold species (with temperature of ∼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 < ω < 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+, = For instance, for ≪0.5%, essentially all the wave energy passes through the resonances to reach the ionospheric boundary. For ηO+ = 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.