In other words, ion wave propagates due to two reasons: ion thermal motion thatmakes it similar to the sound wave in neutral gas and presence of the electric field in a quasi -neutral plasma that
couples ions and electrons motion.
Note that we used a quasi-neutrality condition to derive the ion wave. If quasi-neutrality condition is relaxed and thus Poisson equation is used to calculate the electric field, one can obtain the
modified dispersion relation:
Eq. (1.112)) and electric field due to electrons (first term in Eq. (1.112)). The sound speed depends
on electron temperature since electric field is proportional to it and ion mass since ion inertia
depends on it.