Calculated densities of H₃O⁺(H₂O)_n,NO₂^- (H₂O)_n, CO₃^- (H₂O)n and electron in the nighttime ionosphere of Mars: Impact of solar wind electron and galactic cosmic rays

Abstract

We have calculated the densities of positive ions and negative ions in the ionosphere of Mars at solar zenith angle 106° between height interval 0 km and 220 km. This model couples ion-neutral, electron neutral, dissociation of positive and negative ions, electron detachment, ion-ion, ion-electron recombination processes through 117 chemical reactions. Of the 34 ions considered in the model, the chemistry of 17 major ions (O₂ ⁺, NO⁺, CO₂ ⁺, H₃O⁺H₂O, H₃O⁺(H₂O)₂, H₃O⁺(H₂O)₃, H₃O⁺(H₂O)₄, O₂ ⁺CO₂, H₃O⁺, CO₄ ^-, CO₃ ^-, CO₃ ^-H₂O, CO₃ ^- (H₂O)₂, NO₂ ^-H₂O, NO₂ ^- (H₂O)₂, NO₃ ^-H₂O, and NO₃ ^- (H₂O)₂) are discussed in this paper. At altitude below 70 km, the electron density is mainly controlled by hydrated hydronium ions and water clusters of NO₂ ^- and CO₃ ^-. The ions O₂ ⁺ and NO⁺ dominate above this altitude. This calculation suggests that the ionosphere of Mars contains F and D peaks at altitude ~130 km and ~30 km due to precipitation of solar wind electron and galactic cosmic rays respectively. F peak is mainly produced by O₂ ⁺ after heavy loss of CO₂ ⁺ with atomic oxygen. D peak occurs due to high efficiency of electron attachment to Ox molecules, which entails that concentration of negative ions is higher than that of electron below 30 km. These results are compared with radio measurements made by Mars 4 and Mars 5 in the nighttime ionosphere.