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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 192 Hays (1979) with a full three-dimensional model. The generation of these fields and their mapping into the global electrical circuit will be discussed in Chapters 14 and 15 (this volume). The dramatic increase in middle-atmosphere conductivity during a major solar-proton event causes large changes in the local electric fields at high latitudes. Holzworth and Mozer (1979) carried out balloon measurements of the stratospheric electric field over northern Canada during the intense event of August 1972 and reported a decrease in the vertical field by more than an order of magnitude. The decrease closely paralleled the increase in solar-proton flux and could be explained qualitatively by conservation of the fair-weather current in the presence of the greatly enhanced conductivity. The upward mapping of the thunderstorm-generated fields of the lower atmosphere is sensitive to changes in middle-atmosphere conductivity, since the middle atmosphere represents a low-resistance load to the generator even in quiet conditions. The downward mapping of electric fields generated in the ionosphere and magnetosphere, however, is much less sensitive, since the conductivity of the middle atmosphere remains much less than that of the lower ionosphere even during a major solar-proton event (see Figure 13.11). The changes in the global electric circuit arising from the August 1972 SPE have been examined in detail by Reagan et al. (1983) and Tzur and Roble (1983), all of whom pointed out the importance in estimating the changes in middle-atmosphere electrical parameters of including the current carried by the precipitating protons themselves in the polar-cap region. Changes in the global circuit, however, arose mainly from the Forbush decrease in galactic cosmic- ray flux that accompanied the event rather than from the solar-proton flux. Measurements of the electric field in the mesosphere have been carried out with rocketborne techniques and have yielded conflicting and unexpected results. Most startling of these is the measurement of strong electric fields in the lower mesosphere (Tyutin, 1976; Hale and Croskey, 1979; Maynard et al., 1981) with intensities that can be orders of magnitude larger than those required to maintain continuity of the fair-weather current. The reality of these fields has been questioned (Kelley et al., 1983), and the possibility that they are instrumental artifacts has not been entirely laid to rest. No satisfactory explanation of their existence, either as a genuine atmospheric phenomenon or as an instrumental effect, has yet been proposed. However, they remain an intriguing feature of the middle atmosphere. The anomalous fields, if they are real, cannot be mapped from above or below, since they are present only in relatively well- defined height ranges. Any plausible explanation must involve either a local mesospheric generation mechanism or a dramatic local decrease in conductivity, in which case the strong electric field would be needed to maintain current continuity. The latter possibility appears to be ruled out by simultaneous conductivity measurements made on the same flight (Maynard et al., 1981). These measurements show that the conductivity is indeed low in the region of the strong electric fields but is still large enough to provide a vertical current density about 200 times larger than the fairweather value. The fact that the strong fields are usually seen near the 60- to 65-km height region, where the equalizing layer exists, is a possible clue. In this region the dominant negatively charged current carriers change from the slow-moving negative ions below to the highly mobile electrons above, and a fairly sharp change in electric field must result from the need to conserve vertical current alone. However, as mentioned above, the fields measured are much larger than those associated with this upward mapping process, and there is no obvious reason why strong fields should be generated in this neighborhood. The observations challenge our picture of the middle atmosphere as a passive element in the global electrical circuit and suggest that there may be field-generating mechanisms that we do not yet understand. Even if the electric fields do turn out to be instrumental artifacts, their explanation will contribute to our understanding of the limitations of in situ electric-field measurements in the terrestrial environment. CONCLUSION In this brief review we have summarized our present understanding of the electrical structure of the atmosphere in the 30- to 100-km height range. The sources of ionization in this region are reasonably well known, and their variations in time and space are at least qualitatively understood. The complexities of the ion chemistry that connects the ionization sources to the ambient ion composition still require a great deal of unraveling. We are still quite ignorant of many aspects, including photodetachment of negative ions and the role of reactive neutral species with extremely low concentrations. These uncertainties lead to corresponding uncertainties in ion concentration and mobility and in such bulk electrical parameters as the conductivity. Direct experimental measurements have led to considerable progress, but they are beset by difficulties of interpretation and by inconsistencies among themselves. Finally, the recent observations of large mesospheric electric fields have raised first-order questions about our understanding of atmospheric field-generating mechanisms or of the in