Photoionisation cross sections of CO₂ and CO producing CO⁺(B²Σ⁺), which are taken from Wu & Judge (1986) and Plummer et al. (1977), respectively. The cross section of Wu & Judge (1986) is linearly extrapolated for the wavelength below 180 Å. The daily-averaged solar flux for the wavelengths below 800 Å, which was obtained from the measurements of the Solar Extreme Ultraviolet Monitor instrument onboard MAVEN on 7 April 2016 for the orbit #2960 (Eparvier et al. 2015), is plotted with a scale on the right y-axis.