Wave Propagation in a Partly Ionized Gas

A three-fluid theory (using Maxwell's equations together with a set of coupled hydrodynamic equations for an interacting mixture of electrons, ions, and neutral molecules) is used to examine small-amplitude oscillations in an infinite, homogeneous, partly ionized gas with a uniform external magnetic field. Dispersion relations are obtained when the uniform magnetic field is either parallel or perpendicular to the direction of wave propagation, and the results are interpreted (in the case of negligible collisional damping) by analyzing plots of phase velocity versus frequency. Comparison is made with the simpler one-fluid theories (the magneto-ionic theory, magnetohydrodynamics, and acoustics), as well as with the two-fluid theory for a fully ionized gas. Collisional damping effects are included in all the dispersion relations and are described in detail for the magneto-ionic and magnetohydrodynamic limits.