Parallel Propagation of Electromagnetic Waves in a Partially Ionized Plasma with Multiple Species

When waves propagate along the magnetic field in a partially ionized plasma with two or more ion species e.g. H+ and O+, such as in the Earth's ionosphere, because of the differences in mass and density, each species responds to the perturbations of electromagnetic fields differently. Furthermore, collisions among the different ions species, between ions and electrons, and between ions and neutrals also affect the wave propagation. With the linear analysis and the assumption of cold plasma, the general dispersion relation of propagation covering all frequencies, from MHD waves to the light propagation, in a medium with arbitrary species of ions, anions and neutrals is derived from the multiple fluids treatment, in combination with Faraday's Law and Ampere's Law including the displacement current. There are several stop bands and characteristic frequencies. For each ion or anion species, there is a resonant frequency at its cyclotron frequency and a cutoff frequency which depends on the mass density of the speciesand and the magnetic field. The waves are strongly damped at the resonant frequencies and become reflective at the cutoff frequencies. With the collisions, the wave propagates slower than the Alfven speed with the frequency below the ion-neutral collision frequency because of an inertia loading process by neutrals. When the collisions are stronger, the resonance is weaker as the cyclotron motion of the ions is disrupted frequently by the collisions. The roles of the collisions played in wave propagation in the stop bands and in wave damping will be discussed.