Circularly polarized Alfvén and ion cyclotron waves in space plasmas

The linear and nonlinear theory of circularly-polarized Alfvén and ion-cyclotron waves in multicomponent plasmas is reviewed. It is shown that minor heavy-ion components play an important role in the dispersive properties of space plasmas. In the nonlinear theory, the presence of minor heavy-ion components leads to a number of new wave couplings which in turn, can lead to new instabilities. These instabilities are of various kinds: modulational, beat wave and decay instabilities. It is shown that the new instabilities can be very efficient in the energy transfer from the pump wave to the heavy-ion species. This can occur through the Landau damping of electro-acoustic daughter waves, and/or resonance absorption of sideband waves. These processes can help us to understand a number of properties observed in high-speed solar wind streams and in the magnetosphere.