Inferring magnetospheric heavy ion density using EMIC waves

We present a method to infer heavy ion concentration ratios from electromagnetic ion cyclotron (EMIC) wave observations that result from ion-ion hybrid (IIH) resonance. A key feature of the IIH resonance is the concentration of wave energy in a field-aligned resonant mode that exhibits linear polarization. These mode-converted waves at the IIH resonance are localized at the location where the frequency of a compressional wave driver matches the IIH resonance condition, which depends sensitively on the heavy ion concentration. This dependence makes it possible to estimate the heavy ion concentration ratio. In this paper, we evaluate the absorption coefficients at the IIH resonance at Earth's geosynchronous orbit for variable concentrations of He⁺ and wave frequencies using a dipole magnetic field model. We find that the resonance only occurs over a limited range of wave frequency such that the IIH resonance frequency is close to but not exactly the same as the crossover frequency. Using the wave absorption and EMIC waves observed from the GOES 12 satellite, we demonstrate how this technique can be used to estimate the He⁺ concentration of around 4% near L = 6.6 assuming electron-H⁺-He⁺ plasma. Key Points We show a method how to infer heavy ion concentration using observed EMIC wave The linearly polarized EMIC waves can be generated near the crossover frequency We estimate that the He concentration is around 4% near L = 6.6