Amplitude of the mutual impedance spectra, simulated by Model 1, expressed in logarithmic scale as 20 log₁₀(|Z(Ω)∕Z₀|) in anti-phased SDL sub-mode (purple points) and in phased SDL sub-mode (black points) for different temperature ratios τ = T_w ∕T_c = 10, 30, 50 and 100, from left to right, and different cold electron density ratios N_c = 0.5, 0.6, 0.7, 0.8 and 0.9 from bottom to top. The Debye length of the warm electron population is fixed to λ_D,w = 1 m. The blue (resp. black) vertical dotted line shows the position of the cold (resp. total) electron plasma frequency.