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During the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission's deep‐dip #2 campaign of 17–22 April 2015, spacecraft instruments observed all of the physical parameters needed to assess the photo‐chemical‐equilibrium (PCE) explanation for ionospheric variability at a fixed altitude (135 km) near the peak of the Martian ionosphere. MAVEN measurements of electron density, electron temperature, neutral CO2 density, and solar irradiance were collected during 28 orbits. When inserted into the PCE equation, the measurements of varying PCE drivers correlated with the observed electron density variations to within instrumental uncertainty levels. The dominant source of this positive correlation was the variability of CO2 densities associated with the longitudinal wave‐2 component of nonmigrating tides in the Martian thermosphere.

This study assesses under what circumstances the Martian ionopause is formed on the dayside, both in regions where there are strong crustal magnetic fields and areas where these fields are small (<30 nT). Multiple data sets from three MAVEN dayside deep dip campaigns are utilized between periapsis and 600-1,000 km, as well as solar wind observations from Mars Express. The ionopause is identified as a sudden decrease of the electron density with increasing altitude and a simultaneous increase of the electron temperature and variability below 400 km. This is a physically robust approach as the electron temperature is a key parameter in determining the structure of the ionospheric profile, and, therefore, also a strong indicator of the ionopause location. We find that 36% (54%) of the electron density profiles over strong (weak) crustal magnetic field regions had an ionopause event. We also evaluate the roles of ionospheric thermal and magnetic pressures on the ionopause formation as well as the presence of solar wind particles, H + , down to the location of the ionopause. We found that the topside ionosphere is typically magnetized at mostly all altitudes. The ionopause, if formed, occurs where the total ionospheric pressure (magnetic + thermal) equals the upstream solar wind dynamic pressure. Moreover, the lower edge of the ionopause coincides with the altitude where the solar wind flow stops: The thermal pressure suffers a significant reduction with increasing altitude and the solar wind proton density has a prominent increase. Plain Language Summary The ionosphere of Mars is the layer of its atmosphere where gases are separated into ions and electrons by solar radiation. The ionopause is the uppermost region where the ionosphere terminates. However, the Martian ionopause is not well-understood because it does not always form, and when it does, it is located over a large range of altitudes, varies rapidly, and is highly structured. This paper does a statistical analysis of the different parameters that play a role in ionopause formation, both over and far from the strong Martian crustal magnetic field regions. The study focuses on observations from the dayside of Mars, and analyzes several data sets from the MAVEN and Mars Express missions. It is found that the ionosphere almost always contains magnetic fields within it and that there is a pressure balance at its upper boundary (the ionopause) between the solar wind and the ionosphere. Moreover, there are more ionopause events far from the surface magnetic field regions than over them. Despite Venus and Mars not having global magnetic fields, their ionospheres can be found in either a magnetized or unmagnetized state depending on the degree to which the solar wind-draped magnetic field is able to penetrate into the ionosphere. This is a well-known scenario for Venus' ionosphere, where thanks to the Pioneer Venus Orbiter (PVO) mission, the ionopause formation is well characterized (Russell & Vaisberg, 1983). The Venusian ionosphere is found to be unmagnetized when...

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The 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.