Solar Cycle Variability of Hot Oxygen Atoms at Mars

The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O ₂ ⁺ ions. These calculations take into account the calculated vibrational distribution of O ₂ ⁺ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O ₂ ⁺ ions, and the spread of the ion and electron velocities. These production rates were next used in a two-stream model to obtain the energy dependent flux of the hot oxygen atoms as a function of altitude. Finally, the calculated flux at the exobase was input into an exosphere model, based on Liouville's theorem, to calculate the hot oxygen densities as a function of altitude in the exosphere and the resulting escape flux. It was found that hot oxygen densities vary significantly over the solar cycle; the calculated densities vary from about 2×10 ³ to 6×10 ³ cm ⁻³ at an altitude of 1000 km. The escape flux also varies from about 3×10 ⁶ to 9×10 ⁶ cm ⁻² s ⁻¹ .