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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 191 time equalizing layer lies nearly 10 km higher than the daytime layer, and the transition between the two should take place rapidly during twilight. Figure 13.10 Model calculations of the daytime and nighttime conductivity of the middle atmosphere during quiet conditions. Figure 13.11 Model calculations of the daytime conductivity of the middle atmosphere during quiet conditions and during an intense solar-proton event. During a solar-proton event, the conductivity profile of the middle atmosphere is greatly changed. Individual events are so variable in ionization rate, however, that a representative SPE-conductivity profile would not be meaningful. Figure 13.11 is simply intended as an illustration of the conductivity profile calculated from the above model for a fairly intense SPE during daytime. The largest fractional increase from the quiet-time conductivity occurs between 50 and 60 km, and the greatly enhanced ionization in this region tends to eliminate, or at least substantially reduce, the steep gradient immediately above. To summarize, neither direct measurements nor modeling studies of the bulk electrical parameters of the middle atmosphere are in a satisfactory condition. The measurements are difficult to make and tend to be beset by problems of interpretation. Models are based on deficient knowledge of several of the important mechanisms and cannot yet give more than estimates of the altitude profiles of such important parameters as the ion concentration and the conductivity. Much remains to be done to place our knowledge on a secure footing. ELECTRIC FIELDS IN THE MIDDLE ATMOSPHERE The normal electric field in the middle atmosphere is a superposition of fields mapped upward from thunderstorm generators in the lower atmosphere and fields mapped downward from magnetospheric and ionospheric dynamo generators. The possible existence of local electric-field generators within the middle atmosphere is a controversial topic and will be discussed briefly later. The mapping of the fair-weather field in the vertical direction has been studied by a number of authors (e.g., Mozer and Serlin, 1969; Park and Dejnakarintra, 1973), and a full three-dimensional model that includes a realistic thunderstorm distribution and surface topography has been constructed by Hays and Roble (1979). The principal component of the middle-atmosphere electric field provided by this tropospheric source is vertically directed and arises from the necessity for continuity of the vertical current. The vertical electric field is thus roughly inversely proportional to the conductivity, and its order of magnitude varies from 10â1 V/m at balloon altitudes to 10â6 V/m at the base of the thermosphere. The horizontal component of the electric field arises from the nonuniform distribution of thunderstorm generators over the Earth and is largely removed by the short-circuiting effect of the equalizing layer in the mesosphere, where the conductivity increases sharply (see Figure 13.10). The attenuation is not complete, however, and the model of Hays and Roble (1979) predicts horizontal electric fields of magnitude up to a few tenths of a millivolt per meter in the lower thermosphere arising from the fair-weather source. The corresponding problem of mapping the ionospheric and magnetospheric electric fields downward through the middle atmosphere to the ground has also been examined by several authors (e.g., Mozer and Serlin, 1969; Volland, 1972; Chiu, 1974; Park, 1976) using simple one-dimensional models and by Roble and