Comparison between modelled CO⁺ first-negative (0–0) band emission limb intensity profiles for different excitation mechanisms, using neutral densities from MCD (left panel) and MGIT (right panel) models as well as the IUVS/MAVEN observation. The dashed, double-dotted orange curve represents the IUVS/MAVEN observed average intensity of CO⁺ first-negative (0–0) band emission on 7 April 2016 and the grey shaded area is its 1σ uncertainty (taken from Stevens et al. 2019). Here Case-A and Case-B are the limb intensity profiles for the total ionisation of CO (photon + photoelectron), which were modelled by increasing the CO density in the MCD and MGIT models by a factor of 3 and 8, respectively. We note that hν and e_ph represent solar photon and suprathermal electron, respectively.