Effects of solar-wind composition on the threshold for plasma instability in the transition region

The effect of a small concentration of solar-wind α particles on the conditions for drift instability in the transition region is investigated. It is shown that, for α-particle concentrations less than about 20%, the proton-electron threshold curve is sufficiently accurate, for the range of relative electron-proton-α-particle drift speeds of interest. Recent transition region plasma measurements are interpreted in terms of the conditions for marginal stability with respect to electrostatic plasma oscillations.