Alfvén wave velocity amplitude vs. time. Left panel: plasma cooled from T₀(t) = 9000 K to 4000 K (red line); partially ionized plasma with T₀ = 5000 K and ξ_i = 0.02 (blue line). Right panel: plasma heated from T₀(t) = 4000 K to 9000 K (red line); partially ionized plasma with T₀ = 5000 K and ξ_i = 0.02 (blue line; k_x = 10^-6 m^-1). The velocity amplitude has been normalized.