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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 216 likely that the global current can be accurately determined from only a few localized measurements, and some other means must be used to determine its value. Localized measurements, however, are needed to determine the range of variability, especially from globally representative stations such as on a mountaintop (Reiter, 1977a). Ionosphere Potential An important parameter for determining the electrical state of the global circuit is the ionospheric potential, which specifies the potential difference between the ground and the ionosphere assuming that the ground is arbitrarily referenced to zero. This quantity can be estimated by using the values of the global resistance, about 200 â¦, and the values of the total current flowing in the circuit, 750 to 2000 A, to get 150 to 400 kV. The ionospheric potential is of fundamental importance for the global circuit because it is one of the few measurable global parameters. It can be determined by integrating the altitude profile of the electric field measured from ascending or descending balloons or aircraft. This technique uses the facts that the electrical conductivity of the ionosphere is so large that any horizontal potential difference is small and the entire ionosphere has a uniform potential difference with respect to the Earth. At high latitudes, however, it is necessary to consider the horizontal potential differences that are generated by the solar-wind/magnetospheric generator discussed later. Another factor is that the electric field decreases exponentially with altitude in such a manner that the product of the electrical conductivity and electric field remains constant with altitude. Therefore, the bulk of the ionospheric potential drop occurs within the first few kilometers above the Earth's surface, a region that is easily attainable by balloons and aircraft. The ionosphere potential can also be determined from measurements of the air-earth current density and electrical- conductivity profiles. MÃ¼hleisen (1977) summarized the distribution of his ionospheric potential measurements made from balloons during the period 1959 to 1970. The minimum measured potential was 145 kV, the mean 278 kV, and the maximum 608 kV. The measurements also showed that the diurnal UT variation of ionospheric potential is similar to the Carnegie curve, shown in Figure 15.3 (top), and that there is an 11-year variation in ionospheric potential that is out of phase with the solar sunspot cycle. Markson (1976, 1977) made aircraft measurements of the ionospheric potential that are shown in Figure 15.7. These measurements have a diurnal UT variation similar to the Carnegie curve and also show the magnitude of day-to-day variability that is associated with the inherent variability of the worldwide thunderstorm generator and, to a smaller extent, the global resistance. Measurements of the ionospheric potential are important for understanding the global electric circuit and should be continued. Figure 15.7 Summary of ionospheric potential measurements as a function of time (UT) of aircraft soundings made by Markson (1976, 1977).