Stability of the Ion-Acoustic Wave in Permeating Plasmas: Application to Comets

The ion-acoustic (IA) wave is a mode that is easily excited when the electron streaming velocity exceeds the ion-acoustic phase speed. We study the stability of this wave when a streaming quasi-neutral electron-ion plasma passes through another quasi-neutral pair-ion electron target plasma. Such a situation occurs when the solar wind (the streaming component consisting of hydrogen (H+) and solar wind electrons (`se')) flows past a comet. The cometary pair-ion electron plasma is composed of positively charged oxygen ions (O+), negatively charged oxygen ions (O-) and cometary electrons (`ce'). All five constituents have been modeled by kappa distribution functions. We find that the growth rate of the IA wave, which occurs under zero-current conditions, decreases with increasing spectral indices of the kappa distributions. The growth rate which increases with increasing O+ and O- densities and electron flow speeds, decreases with increasing hydrogen densities.