Neutral densities, ionization rates, and ionization efficiencies for CO₂ (blue) and O (red) as a function of altitude in the dayside Martian upper atmosphere. The primary and secondary ionization rates are given by the solid and dashed lines, respectively. The upper panels are for the inbound portion of orbit No. 3043 on 2016 April 22 with a solar energy input of 1.2 erg cm⁻² s⁻¹ when integrated over the wavelength range of 0.1–190 nm, and the lower panels for the inbound portion of orbit No. 849 on 2015 March 8 with a larger solar energy input of 1.8 erg cm⁻² s⁻¹. Strong fluctuations in ionization efficiency are seen above 190 km for the latter orbit, likely indicative of substantial energy input from external SW electrons. To reach a clean selection of orbits where secondary ionization is predominantly caused by photoelectrons, all cases showing strong fluctuations in ionization efficiency are excluded from subsequent analyses.