Profiles of various quantities of the H–He atmosphere of KELT-9 b along the y axis directed opposite to the orbital motion of the planet, obtained in simulation with the parameter set N 1 (Table 2 below) and expressed via pressure. Top panel: Population of H I(n2) level in terms of LTE value (red, left axis); ionization degree N_e/N_a (orange) in comparison to Saha equilibrium (turquoise) and type B recombination (blue, formula (D.8)); temperature (gray, upper horizontal axis). Bottom panel: Heating (>0, solid lines) or cooling (<0, dashed lines) terms in the energy equation associated with H I(n1) and H I(n2) levels. Photoionization heating from the H I(n2) level given by the analytical formula $W=n_{{\mathrm{H}\mathrm{I}}_{\mathrm{I}}}(n2)R_{2f}^{\mathrm{p}\mathrm{h}}{\overline{E}}_{2f}^{\mathrm{p}\mathrm{h}}$ is shown with a green line.