Electrostatic high-frequency oscillations in partially ionized two-fluid plasmas

We report the existence of a new electrostatic mode in an electron-neutral collision-dominated, partially ionized plasma, in which electrons and the neutral gas are coupled through collisions to behave as a single electron-neutral (EN) fluid, and positive ions behave as a separate fluid. Interestingly, the EN-ion plasma collectively oscillates with a frequency close to or higher than the ion-plasma oscillation frequency (ωpi), and the corresponding wave mode appears similar to Langmuir waves in classical plasmas. However, when ions form only a stationary background, the EN fluid oscillates at a frequency much lower than ωpi. Estimates suggest that such high-frequency electrostatic modes may be excited in laboratories and in the solar photosphere.