COUPLED GAS ACOUSTIC AND ION ACOUSTIC WAVES IN WEAKLY IONIZED PLASMA

Gas acoustic and ion acoustic modes are investigated in a collisional, weakly ionized plasma in the presence of un-magnetized ions and magnetized electrons. In such a plasma, an ion acoustic mode, driven by an electron flow along the magnetic field lines, can propagate almost at any angle with respect to the ambient field lines as, long as the electrons are capable of participating in the perturbations by moving only along the field lines. The electron-ion collisions are shown to modify the previously obtained angle dependent instability threshold for the driving electron flow. The inclusion of the neutral dynamics implies an additional neutral sound mode which couples to the current driven ion acoustic mode, and these two modes can interchange their identities in certain parameter regimes.