NONLINEAR DECAY OF ELECTROMAGNETIC ION-CYCLOTRON WAVES IN THE MAGNETOSPHERE

We study the parametric decays of left-hand polarized electromagnetic ion cyclotron waves, propagating parallel to the external magnetic fie ld, 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 coup lings. 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-ha nd polarized electromagnetic ion cyclotron waves which has a resonance at the He+ ion gyrofrequency. These instabilities can also play a rol e in the energy transfer from the pump wave to the He+ ions through re sonance absorption, preferably in the direction perpendicular to the e xternal magnetic field. The new couplings give rise to several types o f parametric instabilities such as ordinary decay instabilities, beat wave instabilities, and modulational instabilities. There are also cou plings where the pump wave decays into two electromagnetic sideband wa ves.