Left: R₃₂₁ as a function of kinetic temperature T_kin and density n. The plot shows that the ratio is mostly dependent on temperature and independent of the density of the gas. Right: R₄₀₄ as a function of kinetic temperature T_kin and density n. The plot shows the strong dependence of the ratio on both temperature and density for densities <10⁷ cm^-3. The red plus signs indicate locations where the observed line ratios R₃₂₁ and R₄₀₄ are matched at the same time.