Model sensitivity for the CH₄ fluorescence in some selected spectral intervals. Top panel: sensitivity to T_vib (ν₄) profile (non-isothermal above 25 km vs isothermal at 125.5 K). Other panels: sensitivity to model omitting individual fluorescence contributions one-by-one: ¹³CH₄, 2ν₄–ν₄, 3ν₄–2ν₄, CH₃D ν₃. In each panel, the nominal model including all contributions is shown in blue, and the alternative model in red.