GROWTH-RATES OF NEW PARAMETRIC-INSTABILITIES OCCURRING IN A PLASMA WITH STREAMING HE++

We consider parametric instabilities of a circularly polarized pump Al fven wave, which propagates parallel to the ambient magnetic field; th e daughter waves are also parallel-propagating. We follow Hollweg et a l. (1993) and consider several new instabilities that owe their existe nce to the presence of Streaming alpha particles. One of the new insta bilities is similar to the familiar decay instability, but the daughte r waves are a forward going alpha sound wave and a backward going Alfv en wave. The growth rate of this instability is usually small if the a lpha abundance is small. The other three new instabilities occur at hi gh frequencies and small wavelengths. We find that the new instability which involves the proton cyclotron wave and alpha sound (i.e., the ( +f, alpha s) mode) can be the fastest growing instability if beta appr oximate to 1. However, if beta is small, then the instability which ca n compete with the decay instability is the (+f, -alpha) instability, which involves both the proton and alpha cyclotron resonances, but the pump wave must have low frequency and large amplitude. These instabil ities may be a means of heating and accelerating alpha particles in th e solar wind, but this claim is unproven until a fully kinetic study i s carried out.