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

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.

► Energy loss method used to calculate ion production rates from galactic cosmic rays.
► The ion model couples 35 ions through 6 processes and 101 chemical reactions.
► We find that ion–neutral collisions with rates ∼105 cm−3 s−1 are most important.
► We classify the other 5 processes according to their importance.
► We identify those ions and reactions for which the processes are significant.