In othe r wor ds, ion wav e propagat es due to two reasons: ion thermal motion that mak es it similar to the sound wave in neutral gas and presence of the elect ric field in a quasi -neutral plas ma that
coupl es ions and elect rons motio n.
Note that we used a quasi-ne utralit y condition to derive the ion wave. If quasi-ne utrality condition is relaxed and thus Poisson equation is u sed to cal culate the ele ctric field, one can obtain the
modifi ed disp ersion relat ion:
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.