Electrostatic heating of solar wind ions beyond 0.1 AU

It is argued that the large electron-proton temperature ratio predicted by the Hartle-Sturrock two-fluid solar wind model may be unstable to generation of ion acoustic waves beyond 0.1 AU, and that this instability is capable of providing a turbulent mechanism for the preferential heating of protons to produce ratios Te/Tp ≲ 5–10 near 1 AU in accord with experimental observations.