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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 210 The field-dependent currents and convection currents are distributed over large areas and, in the absence of lightning, tend to vary rather slowly with time. The lightning current occurs impulsively and represents a discontinuous transfer of charge in both space and time. Corona and lightning usually transfer negative charge to the ground, and precipitation and turbulence transfer positive charge. Krider and Musser (1982) showed that the average Maxwell current density is usually not affected by lightning discharges and varies slowly throughout the evolution of the storm. Since the Maxwell current is steady at times when the electric field both at the ground and aloft undergoes large changes in amplitude, and sometimes even polarity, they inferred that the cloud electrification processes are substantially independent of the electric field. Also, since the Maxwell current varies slowly throughout the storm, it probably represents an electrical quantity that is coupled directly to the meteorological structure of the storm. Thus, the thunderstorm appears to be a current source that produces a quasi-static Maxwell current density. Grenet (1947, 1959) and Vonnegut (1953, 1965) proposed a convective thunderstorm generator in which the electrification process is related to the updrafts and downdrafts acting within the thunderstorm. In this generation process positive space charge is convected to the top of the thunderstorm cloud by updrafts within it, and the negative charge attracted to the top of the cloud is swept downward by convective downdrafts at the edges of the cloud. For this generator, the magnitude of the cloud electrification is related to the strength of thunderstorm convective activity. The convection mechanism also involves conduction currents flowing from the upper atmosphere to the top of the cloud. The magnitude of the electrification process can, therefore, increase with an increase in the electrical conductivity of the atmosphere over the cloud. The above discussion reveals the complexity and difficulty involved in modeling the thunderstorm as a generator in the global circuit. Significant progress has been made in recent years, but there is still a long way to go before a generally accepted thunderstorm model will be available for use in a global model of atmospheric electricity. Yet such a model is of prime importance, and research in this area should be pursued vigorously. SOME PROPERTIES OF THE GLOBAL CIRCUIT According to the classical picture of atmospheric electricity (Dolezalek, 1972; IsraÃ«l, 1973), the totality of thunderstorms acting together at any time charges the ionosphere to a potential of several hundred thousand volts with respect to the Earth's surface. This potential difference drives a vertical electric conduction current downward from the ionosphere to the ground in all fairweather regions on the globe. The fair-weather electric conduction current varies according to the ionospheric potential difference and the columnar resistance between the ionosphere and the ground. Horizontal currents flow freely along the highly conducting Earth's surface and in the ionosphere. A current flows upward from a thunderstorm cloud top toward the ionosphere and also from the ground into the thunderstorm generator, closing the circuit. A lumped parameter schematic of the global circuit is shown in Figure 15.1 (b). This schematic does not represent the real circuit but rather illustrates its basic concepts. The global fair-weather load resistance is given as W a and is about 250 â¦. The thunderstorm generator source is shown along with its equivalent internal resistance, W i, which is not well known. The total resistance between the thunderstorm and ionosphere is represented as W 0 and is about 105-106 â¦, and the total resistance between the thunderstorm and ground is represented by W u and is also not well known. Markson (1978), however, suggests that W u is small because of corona discharge beneath the storm, having a value of about 104-105 â¦. Some of the overall properties of the global circuit are summarized in Table 15.1. There are many additional elements that complicate this simplified classical picture, and these are discussed in the following subsections. Global Thunderstorms The thunderstorm generator hypothesis proposed by Wilson (1920) was based on his observations that beneath the thundercloud negative charge is transferred to the Earth and above the thundercloud positive charge is transferred to the conductive upper atmosphere. A subsequent discovery was the close correlation between the diurnal universal time variation of the thunderstorm generator current (represented by the frequency of thunderstorm occurrence) and the load current (represented by the fair-weather ground electric field or air-earth current density), integrated over the surface of the Earth. In about the 1920s the electric field over the oceans was found to vary diurnally in accordance with universal time (Parkinson and Torrenson, 1931), as shown in the upper frame of Figure 15.3. The diurnal change of electrical conductivity over the oceans is relatively small, and therefore Ohm's law requires that the airearth current density also follow the diurnal variation of electric field. The maximum value of both the average
THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 211 electric field and the current over the oceans occurs near 1900 UT, and minimum values near 0400 UT. TABLE 15.1 Some Properties of the Global Circuit Number of Thunderstorms Acting at One Time 1500-2000 Currents above Thunderstorms (A) (a) Range 0.1 to 6 (b) Average 0.5 to 1 Global Current (A) 750-2000 Ionospheric Potential (kV) (a) Range 150-600 (b) Mean 280 Columnar Resistance at Sea Level (â¦/m2) (a) Low latitude 1.3 Ã 1017 (b) High latitude 3 Ã 1017 (c) Tibet and Antarctic plateau 2 Ã 1016 Total Resistance (â¦) 230 (including resistance decrease by mountains) 200 Current Density (A/m2) (a) Inhabited and industrialized areas 1 Ã 10-12 (b) Vegetated ground and deserts 2.4 Ã 10-12 (c) South Pole Station 2.5 Ã 10-12 Potential Gradient (V/m) (a) Equator 120 (b) 60Â° latitude 155 (c) South Pole 71 (d) Industrial areas 300-400 Average Charge Transfer over the Entire World (C kmâ2 yrâ1) + 90 C Total Charge on the Earth (C) 500,000 Electrical Relaxation Times (a) 70 km 10-4 sec (b) 18 km 4 sec (c) 0.01 km 5-40 min (d) Earth's surface 10-5 sec Electrical Conductivity (mho/m) Sea level 10-14 Tropopause 10-13 Stratopause 10-10 Ionosphere (a) Pedersen conductivity 10-4-10-5 (b) Parallel conductivity 10 Whipple and Scrase (1936) obtained the average thunderstorm probability as a function of local time at Kew, England, from corona current records. Assuming that the same thunderstorm probability curve also exists on other continents as a function of local time, they combined this curve with the world thunderstorm day statistics of Brooks (1925) and obtained the diurnal variation of worldwide thunderstorm activity as a function of universal time, shown in the bottom frame of Figure 15.3. The three major component curves with their maxima at 0800, 1400, and 2000 UT represent the contributions of the major thunderstorm regions of Asia and Australia, Africa and Europe, and America, respectively. The summation of the component curves represents the diurnal variation of worldwide thunderstorm activity. The similarity of the diurnal variation of electric field over the ocean and the diurnal variation of worldwide thunderstorm activity supports the hypothesis that thunderstorms are the electrical generator in the global circuit. The maxima and minima of both curves occur at about the same universal times. The amplitudes of modulation for the two curves, however, are different. The amplitude of the electric-field curve is about 20 percent, and the amplitude of the thunderstorm curve is about 45 percent. Whipple and Scrase (1936) suggested that this difference might be resolved if a steady supply current from worldwide ocean thunderstorms is added to the worldwide continental distribution of storms. This suggestion, however, is not supported by current data. The difference in amplitudes probably arises from the enormous variability in thunderstorm electrification. Although the similarity between the diurnal UT worldwide thunderstorm frequency curve and the diurnal UT electric-field curve suggests that thunderstorms