Recent developments and outstanding problems in the theory of the D region

The present understanding of the ionization processes, and of the positive and negative ion chemistry of the D region is reviewed with particular attention being given to outstanding problems. Rocket-borne mass spectrometer observations carried out in the past decade have shown that the change in positive ion composition with height (from NO⁺, O2⁺ and metallic ions above to water cluster ions H⁺· (H2O)n below) is a general feature of the D region at all latitudes and under all geophysical conditions. Furthermore, these observations indicate that this change occurs over a very small range of heights, between 82 and 86 km under normal conditions and at lower heights during auroral or Polar Cap Absorption conditions. These results are discussed in terms of current ideas concerning the formation of water cluster ions from O2⁺ and NO⁺ ions. Mass spectrometer observations by two experimental groups have provided conflicting results concerning the negative ion composition in the D region and cannot be used to test theoretical models. Such models have been derived from laboratory measurements, and involve reaction schemes leading from O2⁻ ions, formed by electron attachment, to ions O3⁻, O4⁻, CO3⁻, CO4⁻, NO2⁻ and NO3⁻. The effect on these models of including recent laboratory measurements of negative ion hydration is considered, and the importance of photodetachment is also mentioned.