Parametric decay instability in high‐speed solar wind streams

The parametric decay of an ion cyclotron pump wave in the high‐speed solar wind streams is studied. The pump wave is an exact stationary solution of the Vlasov equation and Maxwell's equations, and it decays parametrically into an ion acoustic and a product ion cyclotron wave. The case where all the waves propagate parallel to the uniform interplanetary magnetic field is considered. Two typical measurements of the high‐speed solar wind protons provide the input data for the calculation of the parametric decay instability. Although this instability has been considered important for the Alfvén wave, the calculation shows that it is probably not important for the ion cyclotron wave, since the range of wave number values where the growth rate is above threshold is very small.