Observations by the EISCAT and Millstone Hill radars of strongly enhanced, often asymmetric, 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. ¿ American Geophysical Union 1992