Left panel: slow-wave period vs. time for plasma cooled from T₀(t) = 9000 K to 4000 K (red line); almost fully ionized plasma with T₀ = 8000 K and ξ_i = 0.99 (blue line); almost neutral plasma with T₀ = 5000 K and ξ_i = 0.02 (black line). Right panel: slow-wave damping time vs. time for plasma cooled from T₀(t) = 9000 K to 4000 K (red line); almost fully ionized plasma with T₀ = 8000 K and ξ_i = 0.99 (blue line); almost neutral plasma with T₀ = 5000 K and ξ_i = 0.02 (black line; k_x = 10^-6 m^-1).