Expected effect of photoelectrons on ionisation and heating rates. Top panel: fractional ionisation x_e as a function of neutral number density n_H I when a Jovian-like planet is irradiated by a solar spectrum. Five particular altitudes are labelled by coloured squares and correspond to the series of panels below. Bottom left panels: contribution to the photoionisation rate at different altitudes, without secondary ionisation J x_H I [s⁻¹] (solid lines) and with photoelectrons (dotted lines), as a function of wavelength. Bottom right panels: contribution to the heating rate at different altitudes, H′ [erg s⁻¹ cm⁻³] representing the energy available before photoionisation (solid lines). The dashed line represents the heating rate due to photoionisation without the effect of photoelectrons, and the dotted lines show the net heating rate with secondary ionisation. The dashed and dotted lines overlap at higher altitudes. The curves provide valuable insight into the wavelengths that contribute to heating at each altitude and how this energy is affected by photoelectrons.