CH₄ and CH₃D fluorescence. Observations (in black) are compared to best model fits (in blue). The model includes contributions from the ν₄, 2ν₄–ν₄, and 3ν₄–2ν₄ bands of CH₄, and from the ν₆ and ν₃ bands of CH₃D (see text and Appendix A for details), as well as a solar reflected component with the I/F reflectivity shown in Fig. 1. CH₄ and ¹³CH₄ (resp. CH₃D) entirely dominate the emission in the upper (lower) panels.