  Some essential features of the ion plasma wave in both kinetic and fluid
descriptions are presented. The wave develops at wavelengths shorter than the
electron Debye radius. Thermal motion of electrons at this scale is such that
they overshoot the electrostatic potential perturbation caused by ion bunching,
which consequently propagates as an unshielded wave, completely unaffected by
electron dynamics. So in the simplest fluid description, the electrons can be
taken as a fixed background. However, in the presence of magnetic field and for
the electron gyro-radius shorter than the Debye radius, electrons can
participate in the wave and can increase its damping rate. This is determined
by the ratio of the electron gyro-radius and the Debye radius. In
interpenetrating plasmas (when one plasma drifts through another), the ion
plasma wave can easily become growing and this growth rate is quantitatively
presented for the case of an argon plasma.
