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Statistically ion and electron densities are enhanced above strong crustal magnetic field regions according to measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. Plasma created by ionization of neutrals in the lower ionosphere (where chemistry dominates) flows upward and becomes trapped on closed magnetic field loops. Enhanced ion density in the ionosphere (particularly O2+) is associated with enhanced photochemical escape of atomic oxygen. This paper presents a quasi‐1D multi‐fluid time‐dependent model of the Martian ionosphere for nine ion species. Ionospheric temperatures are adopted but ion densities and velocities (along the field lines) are determined using a numerical solution of the continuity and momentum equations. Diurnal effects are explored by varying photoionization rates. Three crustal field cases are considered: a low altitude closed, a high altitude closed, and a high altitude open field line. Additionally, a case with no crustal field is modeled to provide a comparison between regions with and without crustal fields in the upper Martian ionosphere. Model results show higher ion and electron densities in the crustal field cases than in the purely induced field case. Additionally, we find that densities are generally higher on the closed field lines than on the open field lines, and ion velocities are generally up the field lines, away from the Martian surface. We also find that velocities are larger on the open field line case. We compare modeled density results to MAVEN data and find general agreement. Implications for atmospheric escape, particularly photochemical escape of O, are also discussed. Plain Language Summary: Mars may not have a global magnetic field like the Earth, but it does have magnetic sources in its crust, giving rise to localized crustal magnetic fields. Previous spacecraft missions have measured increased ion and electron densities over regions where these crustal fields are strongest. This paper presents a model of ion flow along crustal fields at Mars, to try to explain the enhancement seen in data. Ion densities and velocities along crustal field lines are calculated. A diurnal (day to night) cycle is implemented to see how densities and velocities change throughout a Mars day. Multiple crustal field cases are modeled, as well as a generic "non‐crustal" ionosphere case. Modeled results show higher ion and electron densities in the crustal field line cases when compared to the non‐crustal case. Differences between modeled cases are compared with data results, and general agreement is evident. The effects of the modeled crustal fields on atmospheric escape are also discussed. Key Points: Plasma flows along Martian crustal magnetic field lines are modeled for nine ion speciesPlasma densities are higher for closed crustal magnetic fields than for open or induced fieldsUpward ion flow speeds are greater on open than on closed crustal magnetic fields [ABSTRACT FROM AUTHOR]

We report on the first observations of atmospheric occultations at Mars of ~10 keV X‐rays from an extrasolar source identified as Scorpius X‐1. The measurements are taken by the SEP (Solar Energetic Particle) instrument on the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. The detected X‐ray photons from Scorpius X‐1 penetrate the Martian atmosphere down to ~70 km altitude, before being attenuated by the neutrals in the atmosphere. The occultation altitude varies by a few km depending on the source X‐ray energy and the atmospheric neutral density. In this work, we study the detection response of SEP to Scorpius X‐1 X‐rays and demonstrate that X‐ray occultation data can be used in conjunction with a model of the light extinction curve in order to gain insights into the neutral density of the Mars atmosphere in the 50–100 km altitude range, an important and largely unexplored altitude range at Mars. Key Points: MAVEN/SEP at Mars detects 10–20 keV hard X‐rays from Scorpius X‐1, an extrasolar source ~9,000 light years away from the solar systemThe energy and field of view response of SEP to the measured X‐rays is studied and an X‐ray occultation case‐study is presentedThe X‐ray occultations can be used to gain insights into the neutral density of the Mars atmosphere in the 50–100 km altitude range [ABSTRACT FROM AUTHOR]

Ion loss from the topside ionosphere of Mars associated with the solar wind interaction makes an important contribution to the loss of volatiles from this planet. Data from NASA's Mars Atmosphere and Volatile Evolution mission combined with theoretical modeling are now helping us to understand the processes involved in the ion loss process. Given the complexity of the solar wind interaction, motivation exists for considering a simple approach to this problem and for understanding how the loss rates might scale with solar wind conditions and solar extreme ultraviolet irradiance. This paper reviews the processes involved in the ionospheric dynamics. Simple analytical and semiempirical expressions for ion flow speeds and ion loss are derived. In agreement with more sophisticated models and with purely empirical studies, it is found that the oxygen loss rate from ion transport is about 5% (i.e., global O ion loss rate of Qion ≈ 4 × 1024 s−1) of the total oxygen loss rate. The ion loss is found to approximately scale as the square root of the solar ionizing photon flux and also as the square root of the solar wind dynamic pressure. Typical ion flow speeds are found to be about 1 km/s in the topside ionosphere near an altitude of 300 km on the dayside. Not surprisingly, the plasma flow speed is found to increase with altitude due to the decreasing ion-neutral collision frequency. [ABSTRACT FROM AUTHOR]