  In plasmas with electron drift (current) relative to static ions, the ion
acoustic wave is subject to the kinetic instability which takes place if the
directed electron speed exceeds the ion acoustic speed. The instability
threshold becomes different in the case of one quasi-neutral electron-ion
plasma propagating through another static quasi-neutral (target) plasma. The
threshold velocity of the propagating plasma may be well below the ion acoustic
speed of the static plasma. Such a current-less instability may frequently be
expected in space and astrophysical plasmas.
