Height dependence of atmospheric parameters according to FAL93-3 model (Fontenla et al. 1993). Upper panel: the plasma temperature. Middle panel: blue solid line – the ratio of neutral hydrogen and electron number densities, n_H/n_e; green dashed line – the ratio of neutral helium and electron number densities, n_He/n_e. Lower panel: the ratio of neutral helium and neutral hydrogen number densities, n_He/n_H.