Abnormal Dawn–Dusk Asymmetry of Protonated Ions in the Martian Ionosphere

Abstract

Abstract Normally, the Martian ionosphere displays a dusk enhancement due to continuous depletion of plasma via recombination during day-to-night transport. Using the extensive measurements made by the Neutral Gas and Ion Mass Spectrometer on board the Mars Atmosphere and Volatile Evolution spacecraft, we show that several species, including , OH+, H2O+, and NH+, present instead an abnormal dawn enhancement above the exobase where they are mainly produced by ion-neutral reactions involving H2. Such a peculiarity is indicative of a dawn bulge of H2 present in the Martian upper atmosphere and corona, which is driven by subsidence in regions of horizontal wind convergence and the subsequent buildup of minor atmospheric species with large vertical scale heights. A similar dynamical process is also known to occur in the upper atmospheres of other solar system bodies such as the Earth, Venus, and Titan. Interestingly, despite that the diurnal variations of O and N are subject to the same dynamical effect, a dawn enhancement is not seen for O+ and N+, possibly due to the nonthermal production of their parent atoms in the ambient atmosphere via processes such as photodissociation and dissociative recombination. The H2 distribution inferred in this study is important for a thorough understanding of hydrogen escape and climate evolution on Mars.