Measurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0–4 eV), densities can vary by four orders of magnitude, composition varies with altitude, spacecraft charging varies in time and must be measured very accurately, and instrumental effects (e.g., detector dead‐time and background) can be significant. The SupraThermal And Thermal Ion Composition instrument team has recently released a new set of data products that contain density moments of the primary ion species at Mars, including those derived at periapsis, subject to the full suite of calibration factors required. This article discusses the challenges associated with deriving these densities and provides examples of the key caveats that users of the data should be aware of. A preliminary statistical study of this new data set focuses on the structure and variability of Mars' ionosphere, demonstrating that solar zenith angle effects, the crustal magnetic fields, and electron precipitation on the nightside, drive the strongest structural features, consistent with photochemical theory and previous studies. Dayside ionospheric density profiles are highly repeatable below altitudes of 200 km, marking the region where photochemistry and collisions dominate. In the upper dayside ionosphere (altitudes >300–400 km) changes in the solar wind dynamic pressure on timescales of Mars Atmosphere and Volatile EvolutioN's orbit (hr) drive the largest (factors of 1–3) variability in ionospheric density. In contrast variability in ionospheric density peaks between 150 and 250 km altitude on the nightside (factors of 1–2), consistent with electron precipitation driving ionization in this region.

Ion temperature is an important parameter that influences the structure and evolution of atmospheres (Schunk & Nagy, 2009). Ion temperatures affect collision and chemical reaction rates, so precise knowledge of ion temperatures and the chemical processes operating in planetary atmospheres enables a more thorough understanding of composition and structure in the photochemical region (Fox, 2015). Ion temperatures also influence atmospheric dynamics and energetics. Specifically, hotter temperatures allow more ions to overcome gravity and reach altitudes above the exobase, where electromagnetic forces can accelerate ions to escape velocity. Understanding how ions are supplied to the exosphere is important for a complete description of ion escape during the present epoch (Chassefière & Leblanc, 2004), and can illuminate how ion loss might have varied under different conditions at

In this study, we have analyzed Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of fields and plasma signatures associated with an encounter of fully developed Kelvin–Helmholtz (K–H) vortices at the northern polar terminator along Mars' induced magnetosphere boundary. The signatures of the K–H vortices event are: (a) quasi‐periodic, “bipolar‐like” sawtooth magnetic field perturbations, (b) corresponding density decrease, (c) tailward enhancement of plasma velocity for both protons and heavy ions, (d) co‐existence of magnetosheath and planetary plasma in the region prior to the sawtooth magnetic field signature (i.e., mixing region of the vortex structure), and (e) pressure enhancement (minimum) at the edge (center) of the sawtooth magnetic field signature. Our results strongly support the scenario for the non‐linear growth of K–H instability along Mars’ induced magnetosphere boundary, where a plasma flow difference between the magnetosheath and induced‐magnetospheric plasma is expected. Our findings are also in good agreement with 3‐dimensional local magnetohydrodynamics simulation results. MAVEN observations of protons with energies greater than 10 keV and results from the Walén analyses suggests the possibility of particle energization within the mixing region of the K–H vortex structure via magnetic reconnection, secondary instabilities or other turbulent processes. We estimate the lower limit on the K–H instability linear growth rate to be ∼5.84 × 10−3 s−1. For these vortices, we estimate the instantaneous atmospheric ion escape flux due to the detachment of plasma clouds during the late non‐linear stage of K–H instability to be ∼5.90 × 1026 particles/s. Extrapolation of loss rates integrated across time and space will require further work.

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The solar wind interaction with Mars controls the transfer of energy and momentum from the solar wind into the magnetosphere, ionosphere and atmosphere, driving structure, and dynamics within each. This interaction is highly dependent on the upstream Interplanetary Magnetic Field (IMF) orientation. We use in‐situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to identify several prominent features that arise when the IMF is aligned approximately parallel or antiparallel to solar wind flow (conditions known as “radial IMF”). In particular, solar wind protons and alphas are observed to directly penetrate down to periapsis altitudes, while the magnetic barrier forms deep within the dayside ionosphere. The MAVEN observations are consistent with either an ionopause‐like boundary or diamagnetic cavity forming beneath the barrier, as a consequence of the dense cold ionosphere and the absence of significant crustal magnetic fields at this periapsis location. The planetary ions above the magnetic barrier are exposed to solar wind flow and subsequent mass‐loading. The trueV⃗×B⃗ $\vec{V}\times \vec{B}$ (convective electric field or “ion pickup”) force is weak and highly variable during radial IMF. While wave particle interactions and subsequent wave heating contribute to incorporating the heavy planetary ions into the solar wind flow, the solar wind momentum is not fully deflected around the obstacle and is delivered into the collisional atmosphere. Significant ion heating is observed deep within the dayside ionosphere, and observed ionospheric density and temperature profiles demonstrate that these ion energization mechanisms drive significant erosion and likely escape to space.