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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 227 Figure 15.18 (a) Calculated contours of potential (kilovolts) in a dawn-to-dusk cross section across the magnetic polar cap. A 100-kV dawn-to-dusk potential drop is superimposed upon a 300-kV ionospheric potential background. (b) Calculated latitudinal variation of the ground potential gradient (volts per meter) over the magnetic polar cap. Figure 15.19 Normalized diurnal variation of the ground potential gradient as measured in the Arctic and Antarctic by Kasemir (1972) (solid curve), the diurnal potential gradient variation from the Carnegie cruise (dashed curve), and the calculated potential gradient at South Pole Station due to the downward mapping of the ionospheric potential pattern (long/short dashed curve) shown in Figure 15.16. tern is superimposed upon the Carnegie UT variation, it can be seen that the magnetospheric potential tends to supress the amplitude of the Carnegie variation, a result similar to Kasemir's (1972) observations. A similar situation exists at Thule, Greenland, although the predicted amplitude of the magnetospheric potential variation is reduced somewhat because the station is so near the northern geomagnetic pole. There is considerable day-to-day variability associated with the magnetospheric convection potential pattern; however, in the time-average sense the positive and negative variations are both out of phase with the Carnegie UT variation at these stations and may be responsible for the suppressed amplitude of the UT diurnal variation observed by Kasemir (1972). Stratospheric balloon measurements of magnetospheric convection electric fields have been made for a number of years (e.g., Mozer and Serlin, 1969; Mozer, 1971; Holzworth and Mozer, 1979; Holzworth, 1981). Recently D'Angelo et al. (1982) processed over 1200 hours of stratospheric balloon data and correlated the vertical electric field with magnetic activity parameters. During quiet geomagnetic conditions the classical Carnegie curve was reproduced. During more active geomagnetic conditions, however, the dawn-dusk potential difference of the magnetospheric convection pattern was shown clearly to influence the fair-weather field. The above measurements suggest an electrical coupling between the magnetospheric dynamo and the global electrical circuit and indicate a need for more measurements. With the move of the incoherent-scatter radar from Chatanika, Alaska, to Sonderstrom Fjord, Greenland, and the operation of the EISCAT radar from Tromso, Norway, a unique opportunity exists to examine the high-latitude electrical coupling between the upper and lower atmosphere. SOME OUTSTANDING PROBLEMS Component processes within the global circuit have been studied for nearly a century. The ground electric field is the most common measurement, although numerous measurements of the electrical conductivity and air-earth current have also been made. These measurements form the basis of much of our knowledge concerning the influence of local processes on the electrical structure of the global electrical circuit. The measurements of currents and fields over land stations are highly variable, being subject not only to variations of the global generator but also to local meteorological and anthropogenic influences that at times dominate the global electrical variations. Over the oceans the local influences can be less, but considerable averaging is still necessary to derive the global variations.
THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 228 It is unlikely that enough ground-based measurements of electric currents and fields can be made simultaneously to define the instantaneous properties of the global circuit and its temporal variations. A globally representative single measurement is needed to define the characteristics of the global circuit. A measurement that has been used by a number of investigators to define the state of the global circuit is the ionospheric potential. This value is derived from the height integral of the vertical electric-field profile that is measured either by ascending and descending balloons or by aircraft. An inherent property of the measurement is that the ionospheric potential is nearly uniform over the globe because of the highly conducting Earth's surface and ionosphere. The ionospheric potential is equivalent to the product of the worldwide thunderstorm current output and the total global electrical resistance. It is generally assumed that changes in the global electrical resistance are small and that variations in ionospheric potential reflect the variations in worldwide thunderstorm current output. Only occasional electric-field soundings have been made over the years, and there is a clear need to increase the frequency of such measurements. Another means of obtaining the ionospheric potential is from tethered balloons, as described by Vonnegut et al. (1966) and Holzworth (1984), that have the capability of providing high-time-resolution measurements for global studies. Markson (1978) called for the establishment of the ionospheric potential as a geoelectric index that gives an indication of the state of the global circuit. This index would be the electrical equivalent of the geomagnetic index that has been used over the years to study geomagnetic phenomena within the Earth's atmosphere. The main problem impeding progress in understanding the global circuit is the determination of the current output from thunderstorm generators. There are only a few measurements of the total current flow from storms, and there is a clear need for more measurements to define the current output properties in terms of thunderstorm size, duration, lightning flash frequency, charge separation distance, and other parameters. Recent balloon measurements at stratospheric heights over thunderstorms (Holzworth, 1981) showed prolonged periods (1-3 hours) of dc electric fields reversed from fair-weather conditions, indicating a current flow toward the ionosphere. In addition to single thunderstorm measurements, it is important to be able to obtain information on the global distribution of thunderstorm occurrence. Previous information has been derived from weather stations, when thunder was heard from an observing site (Crichlow et al., 1971), from Schumann resonance (Polk, 1982), and from radio and spheric measurements (Volland, 1984). More recently, lightning detection from satellites has been used to derive information on the global distribution and flash-rate frequency from space, as discussed in the chapter by Orville (Chapter 1, this volume). Krider et al. (1980) designed a ground detection network that detects cloud-to-ground lightning flashes and deployed the detectors in regions that cover large parts of North America. And finally, Davis et al. (1983) examined the feasibility of detecting lightning from a satellite in synchronous orbit. They estimated that three such satellites could provide worldwide coverage. These measurement techniques provide a capability of making progress in the century-old problem of understanding the role of thunderstorms as generators within the global circuit. Simultaneous measurements of the ionospheric potential (either from vertical electric-field soundings or from a tethered balloon), along with measurements of lightning flash frequency (either cloud-to-ground flashes from a ground network or total flash rate from satellites), may determine the degree of synchronization between the two phenomena. Such measurements can be used to monitor the electrical state of the global circuit and also can provide a global indicator to help the understanding various local and regional measurements. These measurements would also be useful for improving our understanding of the role of solar-terrestrial perturbations in altering the properties of the global circuit (Reiter, 1969, 1971, 1972; Markson, 1971, 1978; Cobb, 1978; Herman and Goldberg, 1978; Roble and Hays, 1982). There is also a need to determine the electrodynamic processes operating within the middle atmosphere. According to the classical picture of atmospheric electricity, the middle atmosphere should be passive, yet certain rocket measurements indicate the existence of large electric fields of unknown origin (Bragin et al., 1974; Tyutin, 1976; Hale and Croskey, 1979; Hale et al., 1981; Maynard et al., 1981; Gonzalez et al., 1982). These large electric fields are not understood, and it has been suggested that instrumental effects may be involved (Kelley, 1983; Kelley et al., 1983). It is important to resolve this issue because of the fundamental implications involved in understanding electrodynamic processes within the middle atmosphere. Finally, progress in understanding the global circuit and possible solar-terrestrial coupling mechanisms requires a collaborative effort between observations and theoretical modeling. The measurements are needed to verify model predictions and guide model development, and the modeling results provide a physical constraint for understanding measurements and suggesting various