An experimental test of the electromagnetic ion cyclotron instability within the earth’s magnetosphere

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Abstract
The ATS−6 geostationary satellite has observed many examples of propagating, electromagnetic Alfv́en/ion cyclotron waves in both plasma particle and magnetic field data. These waves have been viewed predominantly near the afternoon and dusk regions of the earth’s magnetosphere with normalized frequencies (ω/ΩH+) ranging between 0.05 and 0.5. Viewed from an average geomagnetic latitude of +10°, the waves have only been observed to propagate northward, suggesting that they are generated within the equatorial or minimum ‖B‖ regions. Two wave events have been chosen for detailed analysis. Both events appeared coincidentally with the encounter of cool plasma populations (5 eV) which joined the hot populations already present (10–40 keV). These coincidences suggest the popular, yet largely untested, electromagnetic ion cyclotron instability as the wave generation mechanism. As a test of this hypothesis, ion cyclotron amplification profiles are obtained by evaluating the linear growth rate integrals under the measured, anisotropic hot ion distributions. The measured frequencies for both of the chosen events are in good agreement with the quite restricted values which correspond to the peaks of the amplification profiles. As a result of magnetic field inhomogeneities, the interactions remain within the linear regime.