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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 185 belts also contributes to middle-atmosphere ionization but in a manner that is highly variable in both latitude and time. During major electron precipitation events this can become the dominant source of ionization above 70 km for brief periods, and ionization rates can be as high as 105 cm-3 sec-1 above 80 km (Reagan, 1977). Vampola and Gorney (1983) deduced zonally averaged ionization rates due to energetic electron precipitation at several magnetic latitudes. Maximum ionization rates occur between 80 and 90 km and vary between about 0.7 cm-3 sec-1 at 45Â° and 6 cm-3 sec-1 at 65Â° latitude. At the higher latitudes energetic electrons are competitive with solar Lyman-alpha as an ionization source even in daytime. They are probably the dominant source above 70 km at night, when the main competitor is photoionization of NO by the weak Lyman-alpha radiation scattered from the Earth's hydrogen geocorona (Strobel et al., 1974). Figure 13.2 Distribution and intensity of solar energetic-particle events, 1956-1973. The peak absorption in the upper part is a measure of the intensity of the polar-cap absorption (PCA) events resulting from high-latitude ionization in the mesosphere; the lower part shows the intensity of the cosmic-ray (CR) increases recorded by neutron monitors and caused by solar-particle-induced nuclear reactions in the lower stratosphere. Bremsstrahlung x rays, generated by the energetic electrons, ionize weakly at heights below 60 km (Luhmann, 1977; Vampola and Gorney, 1983) but are probably rarely competitive with cosmic rays as a global ionization source. ION CHEMISTRY IN THE MIDDLE ATMOSPHERE The principal primary positive ions produced in the middle atmosphere are , , and NO+, all of which participate in a wide range of ion-molecule reactions that lead to a rich spectrum of ambient ions. An equally rich spectrum of negative ions is generated by reactions that are initiated by the attachment of electrons to form the main primary species O2- and O-. In this section the current state of our knowledge of this ion chemistry and of the steady- state ion composition that it produces are discussed. More detailed treatments can be found in review articles by Ferguson et al. (1979) and Ferguson and Arnold (1981). Positive Ions The first measurements of positive-ion composition in the mesosphere were made by a rocketborne mass spectrometer in 1963 (Narcisi and Bailey, 1965). The dominant species below the mesopause were found to be proton hydrates, i.e., members of the family H+ (H2O)n, with a sharp transition at about the mesopause to such simple species as , NO+, and several metallic species, probably of meteoric origin. Many subsequent measurements have verified these results and have shown that the size spectrum of the proton hydrates is very temperature sensitive. At the cold high-latitude summer mesopause, as many as 20 water molecules have been seen clustered in individual ions (BjÃ¶rn and Arnold, 1981). The currently proposed positive-ion reaction scheme leading from the primary ions to the proton hydrates is illustrated in Figure 13.3. Since N2+ is rapidly converted into by charge exchange with O2, the two primary ions of concern are and NO+. The chain that converts O2+ into the proton hydrates was identified by Fehsenfeld and Ferguson (1969) and Good et al. (1970) and is fairly straightforward. Clustering of O2+ to O2 forms O4+, which rapidly undergoes a switching reaction in which the O2 molecule forming the cluster switches with an H2O molecule to form O2+ (H2O). When they are energetically allowed, such switching reactions are usually fast, occurring at virtually every collision between the two species. Subsequent collisions with water molecules lead rapidly to the proton hydrates. The failure of this mechanism above the mesopause is probably due to a combination of factors: the decreasing water-vapor concentration, the increasing electron concentration leading to shorter ion lifetimes against recombination, and the increasing concentration of atomic oxygen. The latter attacks the O4+ clusters through the reaction The chain of reactions leading from NO+ to the proton hydrates is less certain but probably involves several