Plasma-neutral coupling allows electrostatic ion cyclotron waves to propagate below ion cyclotron frequency

The effect of ion-neutral collisions on the propagation characteristics of electrostatic ion cyclotron (EIC) waves in a partially ionized plasma is investigated. The dispersion relation of EIC waves is derived using a fluid model taking neutral dynamics into account. The propagation properties of EIC modes, including the damping factor, are examined for various ionization degrees and collision frequencies, which determine the momentum transferred from ions to neutral particles. It is found that the motion of neutral particles driven by plasma-neutral coupling leads to an increase in the effective ion mass, and consequently, EIC waves can propagate even below the ion cyclotron frequency. In a hot neutral gas, the gas-thermal mode can also propagate as well as the EIC mode. The possibility of observing in the laboratory and the Earth's ionosphere is discussed.