where V
s is the acoustic velocity. It is interesting to note that ion wave still exist even if ion temperature Ti 5 0. This can be understood as follows. In ion oscillations, electrons are not fixed; electrons are pulled along with ions shielding the electric field that can arise from ion motion.
However, shielding of the electric field is not perfect and an electric field about kTe/e can leak due
to electron thermal motion as it was discussed above. As an ordinary sound wave, ions form
regions of rarefaction and compression as shown schematically in Figure 1.21. The compression
region expands into the rarefaction region due to ion thermal motion (this is the second term in
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.