Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km.The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km.Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements.A paramount consideration was to keep the models simple enough to be used conveniently.Available observations are reviewed. Tables are provided for density, temperature, composition (C0 2, 0, CO, He, N, N2, and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day-and nightsides.Estimates are made of other species, including 02 and D.Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, 0, C, and He) as well as the hot components (H, N, C, 0) on the day-and nightsides.1.

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An important mechanism in the generation of polar wind outflow is the ambipolar electric potential which assists ions in overcoming gravity and is a key mechanism for Terrestrial ionospheric escape. At Mars, open field lines are not confined to the poles, and outflow of ionospheric electrons is observed far into the tail. It has thus been hypothesized that a similar electric potential may be present at Mars, contributing to global ionospheric loss. However, no direct measurements of this potential have been made. In this pilot study, we examine photoelectron spectra measured by the Solar Wind Electron Analyzer instrument on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Mars Scout to put an initial upper bound on the total potential drop in the ionosphere of Mars of Φ♂0.3em0.3em≾⊥0.3em0.3em2V , with the possibility of a further ≾4.5 V potential drop above this in the magnetotail. If the total potential drop was close to the upper limit, then strong outflows of major ionospheric species (H+, O+, and normalO2+) would be expected. However, if most of the potential drop is confined below the spacecraft, as expected by current theory, then such a potential would not be sufficient on its own to accelerate normalO2+ to escape velocities, but would be sufficient for lighter ions. However, any potential would contribute to atmospheric loss through the enhancement of Jeans escape.

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