The Neutral Gas and Ion Mass Spectrometer of the Mars Atmosphere and Volatile Evolution provides a large data set to explore the ion composition and structure of the Martian ionosphere. Here the dayside measurements are used to investigate the minor ion density profiles with distinctive peaks above 150 km, revealing a systematic trend of decreasing peak altitude with increasing ion mass. We specifically focus on a subset of species including O$^+$, N$_2^+$/CO$^+$, C$^+$, N$^+$, He$^+$, and O$^{++}$, all of which are mainly produced via direct photoionization of parent neutrals. Our analysis reveals weak or no variation with solar zenith angle (SZA) in both peak density and altitude, which is an expected result because these ion peaks are located within the optically thin regions subject to the same level of solar irradiance independent of SZA. In contrast, the solar cycle variations of peak density and altitude increase considerably with increasing solar activity, as a result of enhanced photoionization frequency and atmospheric expansion at high solar activities. He$^+$ serves as an exception in that its peak density increases towards large SZA and meanwhile shows no systematic variation with solar activity. The thermospheric He distribution on Mars should play an important role in determining these observed variations. Finally, the peak altitudes for all species are elevated by at least several km within the weakly magnetized regions, possibly attributable to the suppression of vertical diffusion by preferentially horizontal magnetic fields in these regions.

Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed when the ambient magnetic fields are more horizontally inclined and under lower Solar Wind (SW) dynamic pressures. We also find that the occurrence increases with increasing atmospheric CO$_2$ density but this trend is restricted to low altitudes and within weakly magnetized regions only. These observations suggest that the formation of electron depletions is two folded: (1) Near strong crustal magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; (2) In weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered but with an intensity substantially reduced at low altitudes due to inelastic collisions with ambient neutrals. In addition, our analysis reveals that both the ionospheric plasma content and thermal electron temperature are clearly reduced in regions with depletions than those without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere.