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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 222 creasing geomagnetic latitude to about 160 V/m above about 60Â°. The electric field is greatly disturbed in regions under thunderstorms, indicating an upward-directed field. The electric field is not greatly modified by the mountains because of the large grid spacing (about 5Â° in latitude and longitude). The ground electric current, however, is strongly influenced by the mountains, as shown in Figure 15.13(b). The contours of the enhanced fair-weather current flow nearly outline the continental regions, with the largest current flowing into the high mountain areas (e.g., Tibet, Andes, Antarctica, Rocky Mountains). This is primarily due to the larger electrical conductivity, with respect to sea level, that exists on the high mountain peaks and to the decreased columnar resistance over mountains. A comparison with a similar calculation made without mountains reveals that about 20 percent of the total current flows into the high mountain areas. Other features of the model calculations are described in detail by Hays and Roble (1979) and Roble and Hays (1979). Figure 15.13 Contours of calculated (a) ground potential gradient in volts per meter along the Earth's surface and (b) ground current density in amperes per square meter when multiplied by 10â12. (c) and (d) are perspective illustrations of the ground potential gradient and ground current density, respectively. Model Improvements The global models of atmospheric electricity that have been constructed are primarily analytical models that have considerably simplified mathematical prescriptions. These models, nonetheless, provide considerable insight into the electrodynamics of the global circuit. There is a clear need to develop numerical models that allow a more realistic prescription of physical processes. For example, the analytic model of Hays and Roble (1979) assumed an ionosphere with a uniform conductivity and electrical vertical profiles that are represented by two exponential functions and simulates the latitudinal variations of cosmic-ray ion production rates only crudely. A numerical model could adopt a more realistic calculation with latitude and longitude, for example, such as that shown in Figure 15.6, and also allow for a day-night variation of electrical conductivity. Realistic perturbations to the global pattern due to solar-terrestrial influences could then be modeled to determine the magnitude of the global response to such events (Tzur et al., 1983). A numerical model could also be expanded to include the total Maxwell current instead of only electrical conduction currents. Such modifications are important, especially for the middle atmosphere where ambipolar diffusion can alter the distribution of currents and fields as discussed by Tzur and Roble (1983), and for the troposphere, where convection, precipitation, conduction, lightning, and displacement currents can be important in disturbed regions. The main improvements in any global model will come primarily by more accurate parameterizations of electrical processes within the troposphere. The numerical model should include the electrical charge structures and conductivity modifications due to clouds and fog, a prescription of turbulent convective processes within the planetary boundary layer, physical charge-transfer process due to the nature of the Earth's surface (e.g.,