WAVE HEATING OF HE-CYCLOTRON WAVES IN THE MAGNETOSPHERE - HEATING NEAR THE H+-HE+ BI-ION RESONANCE FREQUENCY( BY ELECTROMAGNETIC ION)

We investigate the absorption of electromagnetic ion cyclotron (EMIC) wave energy by He+ in the outer magnetosphere and the subsequent perpe ndicular heating of He+. Plasma models, based on satellite data, are c onstructed to represent conditions at dawn and noon MLT for L > 7. Ray tracing using the HOTRAY program shows that EMIC waves generated by a n 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+ concentrat ion is sufficiently small, typically < 1%. Growth and absorption occur on one pass across the equator and do not require reflection. The amo unt of wave absorption decreases with decreasing density, but the amou nt of energy absorbed per particle remains comparable over a range of densities > 2 x 10(6) m(-3). The energy absorbed per particle is small er for very low densities similar to 5 x 10(5) m(-3) due to a smaller path integrated wave gain. In a dipole magnetic field the absorption o ccurs in a localized region of space at latitudes of \lambda(m)\ = 20 - 30 degrees. At noon MLT strong wave growth occurs at lower frequenci es consistent with observations. Wave absorption at the bi-ion frequen cy is also stronger at lower frequencies. When a nondipole magnetic he ld is included to represent magnetospheric compression, it is found th at 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 t he spectral peak shifts to lo Ner frequencies above the equator and th at this effect is most pronounced in a dipole field. We suggest that a bsorption at the bi-ion resonance frequency is responsible for most of the X-type pitch angle distributions of He+ detected by the Active Ma gnetospheric Particle Tracer Explorer (AMPTE) spacecraft.