Effects of heavy ions on the quasi‐linear diffusion coefficients from resonant interactions with electromagnetic ion cyclotron waves

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Abstract
Ion composition measurements provided by recent satellite missions have confirmed the presence of heavy ions in the terrestrial magnetosphere. In order to describe the resonance of energetic ring current particles with electromagnetic ion cyclotron (EMIC) waves in a more realistic terrestrial environment, general expressions are derived that provide quasi‐linear diffusion coefficients in a cold plasma containing heavy ions. Cold plasma theory is used as a first approximation. In such plasma, EMIC waves do not propagate in the frequency range between the ion gyrofrequency and the cutoff frequency for each ion component but form multiple stop bands. No interactions occur within the stop bands and the diffusion coefficients are zero over the corresponding frequency intervals. For most of the wave frequencies of interest, the particles in a multicomponent plasma resonate at lower parallel energies than particles in an electron‐proton plasma for a given harmonic value. Therefore resonance with a fixed frequency wave occurs at larger pitch angles (lower parallel energies) in a multi‐ion than in a proton‐electron plasma. As a direct consequence, pitch angle diffusion coefficients for a given energy decrease at small pitch angles and increase at large pitch angles as heavy ions are added to the plasma. The energy and mixed diffusion coefficients change correspondingly. Also, higher harmonics need to be included in the calculations for resonances at higher energies. The pitch angle diffusion lifetimes are calculated for given plasmaspheric and wave parameters corresponding to conditions at a radial distance L=4. The values of the diffusion lifetimes decrease at low energies and increase at high energies in a multi‐ion as compared to an electron‐proton plasma. As a result, the resonances at lower energies (∼ approximately tens of keV) will contribute to the ion precipitation losses from the ring current during geomagnetic storms.