D ‐region ion chemistry

The status of D ‐region positive‐ and negative‐ion chemistry is reviewed. In the case of the positive‐ion chemistry, the presently accepted ion production rates and ion‐neutral reaction scheme are incompatible with the observed qualitative ion composition measurements. The observations indicate that water‐ion clusters, H 3 O + (H 2 O) n , particularly H 5 O 2 + , are dominant below 80 km, whereas calculations predict that NO + and its hydrates would be dominant. It seems rather likely that an essential element of the physics involved has so far escaped attention. In the case of the negative ions, the few observations only recently obtained are insufficent to critically test the detailed reaction schemes available from laboratory studies. Uncertainties in the concentrations of minor neutral constituents severely limit the ability to predict theoretical negative‐ion profiles at present. The first rocket negative‐ion observations do support the laboratory‐derived prediction of a dominant role for NO 3 − and its hydrates in the D ‐region.