Gas acoustic and ion acoustic waves in partially ionized plasmas with magnetized electrons

The properties of gas acoustic and ion acoustic modes are investigated in a collisional, weakly ionized plasma in the presence of unmagnetized 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. Several effects, including the electron-ion collisions, the perturbations of the neutral gas, and the electromagnetic perturbations, are studied in the present work. 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. The electromagnetic effects, which in the present model imply a bending of the magnetic field lines, result in a further destabilization of an already unstable ion acoustic wave. The applicability of these results to the solar and/or space and laboratory plasmas is discussed.