Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

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AbstractThe structure of the upper atmosphere of Mars provides insights into the physical mechanisms that drive escape of species into outer space. Deviations in plasma density profiles with altitude from the theoretical exponentially decaying formulation have been routinely observed for decades yet remain largely unexplained. Proposed mechanisms driving this variability have focused primarily on plasma‐specific processes, as limited by past plasma‐only observations. The Mars Atmosphere and Volatile Evolution mission's Neutral Gas and Ion Mass Spectrometer data set has recently provided unprecedented planetographic coverage for both ions and neutrals in the Martian upper atmosphere. Ion, electron, and neutral density profiles with altitude, collected on the sun‐lit inbound portion of the spacecraft orbit have been analyzed. It was found that neutral species, measured between ~160 and 200 km, behave consistently with the bulk atmosphere and that variations in ion density profiles follow neutral profile variations at the same altitudes in 70% of the observations. In the remaining 30%, additional structure was apparent in the ionized species' profiles that were found to preferentially lie in regions of strong crustal field or to be measured near dawn. A 1‐D ionospheric model was used to show that many observed features in plasma profiles are directly driven by neutral atmospheric features, providing strong evidence for ion‐neutral coupling in the atmosphere of Mars.