Temporal evolutions of (a) the corresponding H⁺ energy spectrum, (b) the H⁻ energy spectrum, and (c) CO₂ density variation measured by NGIMS during the H-ENA penetration event on 24 April 2018. The altitude of the MAVEN spacecraft is plotted with the dashed red line on the right axis. The weighted average energies obtained from H⁺ and H⁻ count-energy distributions within 4-second and 2-second time windows are represented by the black dots. The horizontal magenta and blue bars at the top of Fig. 2 represent orbits 6938 and 6939, respectively.