Calculated production and loss rates of ions due to impact of galactic cosmic rays in the lower atmosphere of Mars

Abstract

Our understanding of the daytime lower ionosphere of Mars is limited due to lack of observations in this region. We have calculated the production rates, the loss rates and the densities of electrons and 35 ions, in the daytime lower atmosphere of Mars using the energy loss method and the continuity equation controlled by the steady state chemical equilibrium condition. The primary ionization source in the model is taken as galactic cosmic rays. The chemical model couples ion–neutral, electron–neutral, photodissociation of positive and negative ions, electron photo-detachment, ion–ion and ion–electron recombination processes through 101 chemical reactions. The electron density is calculated using charge neutrality condition. Of the 35 ions considered in the model, we discuss in detail the source and sink processes of 20 ions that are most dominant. These are the positive ions H₃O⁺(H₂O)₂, H₃O⁺(H₂O)₃, H₃O⁺(H₂O)₄, H₃O⁺H₂O, H₃O⁺, CO₂⁺CO₂, O₂⁺(CO₂)₂, O₂⁺CO₂, O₂⁺, CO₂⁺ and negative ions CO₃⁻(H₂O)₂, NO₂⁻H₂O, CO₃⁻H₂O, NO₂⁻(H₂O)₂, CO₃⁻, CO₄⁻, NO₃⁻(H₂O)₂, NO₂⁻, NO₃⁻H₂O, O₂⁻. The model calculation suggests that maximum electron density of 0.5×10² cm⁻³ occurs at about 35 km due to high efficiency of electron attachment to Ox molecules, which entails that concentrations of negative ions is higher than that of electron below 35 km. Impact of galactic cosmic rays initially produces CO₂⁺ and O₂⁺ ions, but the ion chemistry eventually leads to the dominance of hydrated positive and negative ions with maximum densities of ∼10³ cm⁻³. It is found that out of all the processes included in the model, the most important process is the ion–neutral collisions wherein the reaction of H₃O⁺(H₂O)_2,3 with water and air molecules having the highest rates of ∼10⁵ cm⁻³ s⁻¹takes place.