Nonlinear decay of electromagnetic ion cyclotron waves in the magnetosphere

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Abstract
We study the parametric decays of left‐hand polarized electromagnetic ion cyclotron waves, propagating parallel to the external magnetic field, in the magnetosphere. We show that the presence of He + ions and a mixed population of thermal and hot protons give rise to new wave couplings. These couplings lead to a number of new instabilities. Some of the instabilities involve sound waves carried mainly by the He + ions, which can be very efficient in heating up the bulk of the He + ions via Landau damping. Other instabilities involve the branch of the left‐hand polarized electromagnetic ion cyclotron waves which has a resonance at the He + ion gyrofrequency. These instabilities can also play a role in the energy transfer from the pump wave to the He + ions through resonance absorption, preferably in the direction perpendicular to the external magnetic field. The new couplings give rise to several types of parametric instabilities such as ordinary decay instabilities, beat wave instabilities, and modulational instabilities. There are also couplings where the pump wave decays into two electromagnetic sideband waves.