Scattering of electromagnetic waves from a plasma: Enhanced ion acoustic fluctuations due to ion‐ion two‐stream instabilities

Observations by the ElSCAT and Millstone Hill radars of strongly enhanced, often asymetric, ion acoustic line spectra in the topside auroral ionosphere have been reported recently by a number of researchers. Such strongly enhanced ion acoustic line spectra are shown to arise naturally in a plasma unstable to the ion‐ion two‐stream instability. A linear theory of density fluctuations is used for the calculations, which should be applicable to weakly unstable non‐magnetized plasmas near the onset threshold of the instability.