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 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.