The role of atomic oxygen concentration in the ionization balance of the lower ionosphere during solar proton events

The influence of atomic oxygen concentration on the height distribution of the main positive and negative ions and on electron density in the mesosphere is studied for the conditions prevailing during the solar proton event on 17 Jan- uary 2005. It is shown by numerical modeling that the elec- tron and ion density profiles are strongly dependent on the choice of the atomic oxygen profile. Experimental measure- ments of the electron density are used as the criterion for choosing the atomic oxygen profile in the mesosphere. With the help of modeling, the atomic oxygen profile in the day- time in the winter mesosphere is found to lead to a model electron density profile best matching the electron density profile obtained experimentally. As a result, with the help of modeling, we find the atomic oxygen profiles at various solar zenith angles in the winter mesosphere which lead to model electron density profiles matching the electron density pro- files obtained experimentally. Alteration of the atomic oxygen concentration leads to a redistribution of the abundance of both positive and negative ion constituents, with changes in their total concentrations and transition heights. In consequence this results in changes of the electron density and effective recombination coeffi- cient. For conditions of low concentration of atomic oxygen (during a solar proton event), the formation of cluster ions is the key process determining electron and ion densities at alti- tudes up to 77 km. The complex negative CO 3 ion is formed up to about 74 km and the final NO 3 ion, which is stable in relation to the atomic oxygen, is the dominant negative ion up to 74 km. As a result the transition heights between cluster ions and molecular ions and between negative ions and elec- tron density are located at 77 km and 66 km, respectively.