Inferring Magnetospheric Heavy Ion Density using EMIC Waves
Authors: Eun-Hwa Kim, Jay R. Johnson, Hyomin Kim, and Dong-Hun Lee
Abstract: We present a method to infer heavy ion concentration ratios from electromagnetic ion cyclotron (EMIC) wave observations that result from ionion 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 found that the resonance only occurs for 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 observed EMIC waves from the GOES-12 satellite, we demonstrate how this technique can be used to estimate
22 that the He+ concentration is around 4% near L=6.6.
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Submitted to: Journal of Geophysical Research
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