Carbon-to-oxygen ratio (C/O, top), atmospheric mean molecular weight, μ (middle), the degree of thermal ionisation, (bottom) for the hot giant gas planet WASP-43b, and the ultra-hot Jupiters WASP-18b, HAT-P-7b, WASP-103b, and WASP-121b. The solar value C/O = 0.54 is plotted in dashed purple (top). The dash-dot purple line shows f_e = 1 × 10⁻⁷ as a threshold for plasma behaviour (bottom). All ultra-hot Jupiters have dayside thermal ionisation f_e > 10⁻⁴ suggesting an extended dayside ionosphere. The detailed results for the individual planets are provided in Figs. A.1–A.8.