Kinetic Theory of Ion Acoustic Waves Observed at Comet 67P/Churyumov-Gerasimenko

This work presents the resonance interaction of ion acoustic waves with the cometary plasma as observed at comet 67P/Churyumov-Gerasimenko. The plasma is comprised of cold, warm, and suprathermal electron populations and water ions such that the quasineutrality is satisfied. The cold electron population is found to play a dominant role in the damping of the waves, and its maximum Landau damping rate is observed when it is 2% of the total electron density in the system. It is determined that lowering the cold electron density supports the current-driven ion acoustic instability at a relatively lower drift speed of the warm and suprathermal electron species. In the absence of cold electrons, the wave phase speed does not change by populating the high-energy suprathermal electron species, therefore, the Landau damping rate of both warm and suprathermal electrons increases by increasing their respective densities. The growth rate of the current-driven ion acoustic instability decreases by elevating the concentration of suprathermal electrons in the case of drifting warm and stationary suprathermal electrons. In the case of stationary warm and drifting suprathermal electrons, the elevated density of suprathermal electron instead favors the instability.