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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 224 to understand local phenomena but also to provide the appropriate guidance for incorporating these effects into global models. Figure 15.15 Contours of log10 J (A/m2), the regional vertical current flow toward the ionosphere from a thunderstorm model considering an ionosphere where (a) the magnetic-field lines are horizontal and (b) the magnetic-field lines are vertical. (Tzur and Roble, 1985b.) ELECTRICAL COUPLING BETWEEN THE UPPER AND LOWER ATMOSPHERE The main generators operating within the Earth's global atmospheric circuit are summarized in Table 15.2. As discussed in the previous section, thunderstorms are generators whose current output maintains a vertical potential difference of about 300 kV between the ground and ionosphere, with a total current flow of about 103 A. The classical picture of atmospheric electricity assumes that the ionosphere is at a uniform potential, and it does not account for either ionospheric or magnetospheric dynamos. The physics of both dynamos are discussed in the chapter by Richmond (Chapter 14, this volume). TABLE 15.2 Generators in the Global Electric Circuit THUNDERSTORMSâcurrent output maintains a vertical potential difference of 300,000 V between ground and ionosphere. Current ~ 103 A. IONOSPHERIC DYNAMOâtides at ionospheric heights maintain horizontal potential differences of 5000-15,000 V between high and low latitudes. Current ~ 105 A. MAGNETOSPHERIC DYNAMOâinteraction of solar wind with Earth's geomagnetic field maintains a horizontal dawn- to-dusk potential drop of 40,000-100,000 V across polar caps. Current ~ 106 A. The ionospheric dynamo is driven by both tides generated in situ and tides propagating upward from the lower atmosphere. These tides generate horizontal potential differences of 5-10 kV within the ionosphere, with a total current flow on the order of 105 A. The magnetospheric dynamo, on the other hand, is driven by the interaction of the solar wind with the Earth's geomagnetic field and generates a horizontal dawn-to-dusk potential drop of typically 40-100 kV across the magnetic conjugate polar cap and a total current flow of 106 A. The magnetospheric convection pattern is Sun-aligned relative to the geomagnetic poles (north geomagnetic pole 78.3Â° N and 291Â° E, south geomagnetic pole 74.5Â° S and 127Â° E), and therefore the pattern remains fixed relative to the Sun but moves in a complex fashion over the Earth's surface as the Earth rotates about its geographic pole. Several empirical models that describe the horizontal ionospheric potential distribution about the magnetic polar cap have been constructed (Volland, 1975, 1978; Heppner, 1977; Sojka et al., 1979, 1980; Heelis et al., 1983). The magnetospheric convective potential distributions are all similar, with a positive perturbation on the dawn side and a negative perturbation on the dusk side of the magnetic polar cap. The main differences between the various models are due to small horizontal-scale structure variations for various levels of geomagnetic conditions. The calculated average potential patterns over the southern hemisphere polar cap for four different universal times (0000, 0600, 1200, and 1800 UT) using the model of Sojka et al. (1980) are shown in Figures 15.16(a)- 15.16(d). Satellite observations have shown that the instantaneous magnetospheric convection pattern is highly variable with considerable small-scale deviations from the mean structure, indicating a turbulent plasma flow. The dawn-to-dusk potential drop across the polar cap varies from about 30 kV for quiet geomagnetic activity, to about 60 kV for average geomagnetic activity, and to about 150-200 kV during geomagnetic storms. In addition, for greater geomagnetic activity the convection pattern expands equatorward by about 5Â° from its normal quiet-time position. The magnetospheric convective electric field is generally confined to the vicinity of the polar cap by shielding charges in the AlfvÃ©n layer of the magnetosphere. However, during rapid changes of magnetospheric convection a temporary imbalance in these shielding charges can occur, and the high-altitude electric fields can cause immediate effects at the magnetic equator at all longitudes (Gonzalez et al., 1979). The observed propagation
THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 225 Figure 15.16 Contours of electric potential (kilovolts) associated with magnetospheric convection at high latitudes in the southern hemisphere for four universal times (a) 0000 UT, (b) 0600 UT, (c) 1200 UT, and (d) 1800 UT. The symbol G represents the geographic pole and GM the geomagnetic pole.
THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 226 may be due to currents either within the magnetosphere or ionosphere (Nopper and Carovillano, 1978) or through the lower-atmospheric electrical waveguide (Kikuchi et al., 1978). The downward mapping of the ionospheric electric fields toward the lower atmosphere has been considered previously by a number of authors (Mozer and Serlin, 1969; Mozer, 1971; Atkinson et al., 1971; Volland, 1972, 1977; Chiu, 1974; Park, 1976, 1979). These studies have all shown that large horizontal-scale electric fields within the ionosphere map efficiently downward in the direction of decreasing electrical conductivity, and that downward electric- field mapping is much more efficient than upward mapping. Both Chiu (1974) and Park and Dejnakarintra (1977b) showed that the anisotropy of the electrical conductivity can have an important influence on the mapping properties of electric fields. Horizontal electric fields of small-scale size ( ~ 1-10 km) are rapidly attenuated as they map downward into the atmosphere from ionospheric heights, but electric fields of larger horizontal scales ( ~ 500-1000 km) map effectively right down to the Earth's surface, as shown in Figure 15.17. Since the electrical conductivity of the Earth's surface is large, horizontal electric fields can usually be neglected, and a vertical electric-field variation results to accommodate horizontal variations of ionospheric potential. Calculations by Park (1976, 1979) and Roble and Hays (1979) showed that the magnetospheric generator can produce perturbations of Â±20 percent in the air-earth current and ground electric field at high latitudes during quiet geomagnetic periods and larger variations during geomagnetic storms and substorms. A calculation that shows the downward mapping of a 100-kV dawn-to-dusk potential drop superimposed on a 300-kV ionospheric potential is given in Figure 15.18(a). The potential perturbation penetrates from the ionosphere down to the tropopause with little attenuation but rapidly decreases within the troposphere to zero at the Earth's surface. The calculated electric field at the Earth's surface is 190 V/m on the dusk side of the polar cap, as shown in Figure 15.18(b). Figure 15.17 Downward mapping factor of the horizontal ionospheric electric field as a function of altitude indicating the magnitude of the attenuation of the electric-field strength for various horizontal scale sizes, Î», in kilometers. Changes in electrical conductivity caused by variations in cosmic-ray ionization during solar-terrestrial events can also change the downward mapping characteristics as discussed by Roble and Hays (1979). In addition, they also have shown that because the magnetospheric potential pattern is Sun-aligned in geomagnetic coordinates, a ground station, balloon, or aircraft at a given geographic location should detect variations organized in magnetic local time. For early magnetic local times the ionospheric potential perturbations of the Earth's potential gradient are positive, and for later magnetic local times the perturbations are negative. At high geomagnetic latitudes these variations are superimposed on the diurnal UT variation of potential gradient maintained by worldwide thunderstorm activity. Kasemir (1972), using data obtained at the South Pole and Thule, Greenland, noted a departure of the diurnal UT variation measured at these stations from the oceanic diurnal electric-field variation measured during the cruises of the ship Carnegie, which is generally accepted as the UT variation due to worldwide thunderstorm activity. The polar curves have a similar shape to the curve derived from the Carnegie cruises but at a much reduced amplitude. From these results Kasemir concluded that another agent besides worldwide thunderstorm activity may modulate the global circuit at high latitudes. The position of the magnetospheric potential pattern over the Earth's surface is shown in Figure 15.16 for four different UTs. It can be seen that the downward mapping of this potential pattern to the Earth's surface gives rise to a complex UT variation due to the displacement of the geographic and geomagnetic poles. The calculated UT variation of the ground electric field at South Pole Station due to the downward mapping of magnetospheric potential pattern is shown in Figure 15.19; the Carnegie UT variation and the Kasemir (1972) measurements are also shown. It is seen that the positive potential perturbation maps down over South Pole Station from about 0200 to 1400 UT, and the negative potential perturbation from 1400 UT to 0200 UT. When this pat