Longitudinal Plasma Waves in the Low‐Frequency Limit

In a partially ionized gas, two longitudinal wave modes propagate at high frequencies, the ion‐acoustic mode and the neutral‐acoustic mode. At very low frequencies, there are also two roots of the dispersion relation, one representing a damped wave and one representing an undamped wave. If the roots are traced as a function of frequency from the high‐frequency regime to the low frequencies, the identification of the roots depends on the ratio of concentrations of ionized and neutral particles. In the low‐frequency limit, either the ion wave is damped or the neutral wave is damped, depending on the value of this ratio. The critical value is derived.