Wave heating of He+ by electromagnetic ion cyclotron waves in the magnetosphere: Heating near the H+‐He+ bi‐ion resonance frequency

Abstract

We investigate the absorption of electromagnetic ion cyclotron (EMIC) wave energy by He+ in the outer magnetosphere and the subsequent perpendicular‐heating of He+. Plasma models, based on satellite data, are constructed to represent conditions at dawn and noon MLT for L > 7. Ray tracing using the HOTRAY program shows that EMIC waves generated by an anisotropic H+ distribution have path integrated gains in excess of 60 dB for the dawn model. The waves undergo strong cyclotron resonant absorption near the bi‐ion resonance frequency when the He+ concentration is sufficiently small, typically < 1%. Growth and absorption occur on one pass across the equator and do not require reflection. The amount of wave absorption decreases with decreasing density, but the amount of energy absorbed per particle remains comparable over a range of densities > 2 × 106 m−3. The energy absorbed per particle is smaller for very low densities ∼ 5 × 105 m−3 due to a smaller path integrated wave gain. In a dipole magnetic field the absorption occurs in a localized region of space at latitudes of |λm| = 20 − 30°. At noon MLT strong wave growth occurs at lower frequencies consistent with observations. Wave absorption at the bi‐ion frequency is also stronger at lower frequencies. When a nondipole magnetic field is included to represent magnetospheric compression, it is found that wave refraction is reduced which results in a substantial increase in the path integrated gain (> 90 dB) and wave absorption takes place at higher latitudes. By considering refraction effects, we find that the spectral peak shifts to lower frequencies above the equator and that this effect is most pronounced in a dipole field. We suggest that absorption at the bi‐ion resonance frequency is responsible for most of the X‐type pitch angle distributions of He+ detected by the Active Magnetospheric Particle Tracer Explorer (AMPTE) spacecraft.