The behavior of the convective growth rate of the electromagnetic proton‐cyclotron instability is investigated in detail in a H+, He+ plasma and when a third (minority) cold ion component such as O+ is taken into account. It is shown that amplification is the result of an interplay between the cold species and the thermal anisotropy of the energetic protons. The results seem to be in good agreement with the observations performed on board GEOS 1 and 2 concerning the generation of ULF waves, below the proton gyrofrequency. It is also shown that a small amount of O+ can strongly affect the behavior of the convective growth rate of the instability below the He+ gyrofrequency, even far away from the O+ stop band.

Preferential acceleration of alpha particles interacting with left‐hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the alpha particle drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.

[1] It is shown that ion-beam plasma electromagnetic right-hand polarized instabilities can be stabilized by nonlinear left-hand polarized waves. This phenomenon constitutes a nonlinear saturation mechanism for right-hand polarized instabilities. Complete stabilization occurs for left-hand wave amplitudes above a threshold value. The saturation mechanism takes place within a large range of pump wave frequencies and plasma temperatures.

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