In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere

Abstract

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‐Situ Measurements of Ion Density in the Martian Ionosphere: Underlying Structure and Variability Observed by the MAVEN‐STATIC Instrument

“…Measurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0-4 eV), densities vary by four orders of magnitude over the altitude range of interest, 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 first half of this article describes the challenges and solutions associated with deriving accurate density moments from measurements made by the MAVEN-STATIC instrument at Mars, accompanying the recent article by Hanley et al (2021) that describes these processes for the derivation of ion temperatures at Mars. Example MAVEN periapsis passes are shown in Figures 1 through 5 with accompanying discussion highlighting the complexities and important caveats that users of the data should be aware of prior to performing scientific analysis on them.…”

MAVEN‐STATIC Observations of Ion Temperature and Initial Ion Acceleration in the Martian Ionosphere

“…In the collisional region below the exobase, the dayside neutral atmosphere is warmer than the nightside (Stone et al, 2018), so ions in the collisional region should also be warmer on the dayside. Hanley et al (2021) first reported dayside ion temperatures significantly hotter than expected, suggesting that an important source of ion energy is missing from current photochemical theory. The same trend is observed using the entire MAVEN data set in Figure 3.…”

“…We have binned the data into 25 km altitude bins and SZA bins of 10°, 20°, or 30°, as indicated in each figure, to investigate how 𝐴𝐴 O + 2 distributions vary depending on these parameters. Given temperature ranges correspond to interquartile ranges of a group of binned measurements, not uncertainties in a particular measurement (typically ∼10% (Hanley et al, 2021)).…”

“…Figure 3 shows median ion temperatures for Maxwellian distributions as a function of altitude and SZA (line color). Ion temperatures are calculated assuming a dominant Maxwellian core using the method described by Hanley et al (2021) for their case studies of temperature profiles. Separating distributions with significant suprathermal components before beginning a statistical analysis reduces systematic errors, which will be discussed in Section 6.…”

“…Data used in this study were collected by MAVEN's SupraThermal And Thermal Ion Composition (STATIC) instrument (McFadden et al, 2015) Hanley et al (2021), which assumes the ions have a dominant Maxwellian component. Distribution functions and temperatures are sampled every 4 s and corrected for spacecraft potential and instrument effects.…”