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The influence of atomic oxygen concentration on the height distribution ofthe main positive and negative ions and on electron density in themesosphere is studied for the conditions prevailing during the solar protonevent on 17 January 2005. It is shown by numerical modeling that theelectron and ion density profiles are strongly dependent on the choice ofthe atomic oxygen profile. Experimental measurements of the electron densityare used as the criterion for choosing the atomic oxygen profile in themesosphere. With the help of modeling, the atomic oxygen profile in thedaytime in the winter mesosphere is found to lead to a model electrondensity profile best matching the electron density profile obtainedexperimentally. As a result, with the help of modeling, we find the atomicoxygen profiles at various solar zenith angles in the winter mesospherewhich lead to model electron density profiles matching the electron densityprofiles obtained experimentally. Alteration of the atomic oxygen concentration leads to a redistribution ofthe abundance of both positive and negative ion constituents, with changesin their total concentrations and transition heights. In consequence thisresults in changes of the electron density and effective recombinationcoefficient. For conditions of low concentration of atomic oxygen (during asolar proton event), the formation of cluster ions is the key processdetermining electron and ion densities at altitudes up to 77 km. The complexnegative CO3− ion is formed up to about 74 km and the finalNO3− ion, which is stable in relation to the atomic oxygen, is thedominant negative ion up to 74 km. As a result the transition heightsbetween cluster ions and molecular ions and between negative ions andelectron density are located at 77 km and 66 km, respectively

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