Atmospheric proflies for ion fraction (panel a), volume heating rate (panel b), mass density (panel c), and temperature (panel d) predicted by model #3 (hydrogen-helium atmosphere, black lines) and model #5 (solar abundances, green lines) for the hot sub-Neptune planet considered in Sect. 5.2 under different levels of XUV irradiation. The different line styles correspond to the different irradiation levels: F_XUV = 1.5 × 10⁶erg s⁻¹ cm⁻² (planet 4.2O in Table A.1; solid lines), F_XUV = 1.8 × 10⁵ erg s⁻¹ cm⁻² (4.2A; dashed lines), F_XUV = 2.7 × 10⁴ erg s⁻¹ cm⁻² (4.2B; dash-dotted lines), and F_XUV = 4.64 × 10³ erg s⁻¹ cm⁻² (4.2C; dotted lines).