Modelling of the ion composition of the middle atmosphere

Present ideas on the ionization and chemical processes which control the positive and negative-ion composition of the middle atmosphere are reviewed. The positive-ion composition of the ionospheric regions is characterized by a change from NO+, O2+ and metallic ions in the E and upper D regions to water cluster ions, H+.(H2O)sub(n), in the middle and lower D region. Theoretical models have provided a reasonable understanding of the molecular-ion results but those concerned with the metal ions are not well developed. The major ions observed at heights below 40 km have been attributed to the replacement of water molecules in H+.(H2O)sub(n) ions by molecules having larger proton affinites, such as CH3CN and CH3OH. Reaction schemes have been formulated but model studies await information on relevant neutral constituents and chemical rate coefficients. The negative-ion composition of the D region has not been firmly established. The presence of heavy ions, particularly above 80 km, has been attributed qualitatively to the attachment of electrons to meteoric debris. At lower heights, recent measurements have indicated that ions HCO3-, CO3- and NO3- predominate and this is predicted by models based on the formation of O2- by attachment followed by reactions with several neutral constituents. At still lower heights, reaction schemes have been formulated to describe the formation of the two families of negative ions observed in recent balloon-borne measurements: NO3-.(HNO3)sub(m) below about 30 km and HSO4-.(H2SO4)sub(n).(HNO3)sub(p) above. As with the positive-ion composition at these heights, model studies require independent information on reactant gases and rate coefficients. The outstanding problems in existing models and the needs for future models of ion composition are emphasised