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ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 176 sphere (e.g., Arnold et al., 1981) have not been detected. This is not consistent with ion-mobility measurements and clearly needs further investigation. AIR-EARTH CURRENT AND ELECTRIC FIELDS IN THE LOWER ATMOSPHERE The atmospheric electric circuit is characterized by a difference in voltage (on the order of 300 kV) between the highly conductive ionosphere (commonly referred to as the equalization layer) and the Earth's surface, which is also a relatively good conductor. This voltage V1 is thought to be maintained principally by thunderstorms acting as the generators and the atmospheric conductivity acting to discharge the ionosphere through continuous flow of current. The value of this air-earth current density in fair-weather areas depends on the voltage VI and the columnar resistance R c and is, according to Ohm's law, The value of the atmospheric electric field E(h) depends on the air-earth current density and the electrical conductivity of the air according to the following Ohm's law relationship: Under steady-state conditions it is expected for reasons of continuity that the air-earth current density is constant with altitude if large-scale horizontally homogeneous conditions exist and if no charged clouds or other disturbances alter the so-called fair-weather conditions. Figure 12.15 shows the atmospheric electric field and both polar conductivities as measured simultaneously over the North Atlantic by Gringel et al. (1978). The field and conductivity profiles show clearly the inverse pattern to each other, which is expected for a constant air-earth current density through the atmosphere. Even a thin cloud layer at 6 km did not disturb this inverse pattern. The mean vertical air-earth current density for this particular balloon flight was calculated to be jÏ = (2.35 Â± 0.15) pA/m2, a typical value for our oceanic measurements. Another profile of jÏ obtained at Laramie, Wyoming (Rosen et al., 1982) to 31 km height is shown in Figure 12.16. Again the profiles of both polar current densities show the expected constancy with altitude. At the same time of this flight at Laramie, the ionospheric potential VI was determined over Weissenau, Germany, by integration of the measured electricfield strength (Fischer and Muhleisen, 1975) and found to be 330 kV, a value that should be the same as over Laramie according to the classical picture of the global Figure 12.15 Polar air-earth current densities ju+ and ju- versus altitude calculated from simultaneously measured electric-field strength E and both polar conductivities (Î» = s) over the North Atlantic (Gringel et al., 1978). electrical circuit. In addition, the columnar resistance was obtained from an atmospheric conductivity profile and found to be Rc = 0.65 Ã 1017 â¦ m2. This implies that the total conduction current density, calculated from V I and R c, is 5.1 pA/m2. The good agreement with the mean value calculated from the conductivity and electric-field profiles proves that the Earth surface and the ionosphere can be regarded as good conductors where charges are distributed worldwide within short times. The fact that j Ï over Laramie shows about twice the value as over the Atlantic is explained by the high altitude of Laramie (2150 m above sea level) resulting in the low R c value given above. These 2 km normally contribute about 50 percent to the total columnar resistance of around 1.3 Ã 1017 â¦ m2 between sea level and the ionosphere. Direct measurements for the air-earth current density in the free atmosphere have been carried out also with long- wire antenna sondes described by Kasemir (1960). Whereas Ogawa et al. (1977) reported a constant air-earth current throughout the troposphere and lower stratosphere, the measurements by Cobb (1977) at the South Pole indicated a slight decrease of the jÏ values
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 177 above the tropopause. The reasons for this current decrease are unknown. At globally representative stations the air-earth current density shows a diurnal variation versus universal time with a minimum at around 0300 GMT and a maximum near 1800 GMT, reflecting the diurnal variations of the ionospheric potential. Figure 12.17 shows this diurnal variation measured near the surface in the North Atlantic (Gringel et al., 1978) and near the ground at the South Pole (Cobb, 1977) during fair weather. The larger variation over the North Atlantic is probably due to the relatively short observation time of only 15 days. The agreement of the two curves also strongly supports the concept of an universally controlled global circuit. Direct-current density measurements near the ground at a continental station have been reported by Burke and Few (1978). They observed a typical sunrise effect that is characterized by a gradual increase of the atmospheric conduction current within an hour after sunrise, reaching a peak about 2 hours after sunrise. The current density then gradually decreased but usually remained at a higher value than was observed before sunrise. This sunrise effect is thought to be caused by mechanical transport of positive charges following the onset of convection. During fog they observed low values of jÏ that can be attributed to an enhancement of the columnar resistance, caused by the attachment of small ions to the fog droplets. Beneath low clouds without precipitation they usually found negative current readings, which are interpreted by Burke and Few (1978) as charge-separation processes occurring in most of the low clouds, whether or not the clouds finally produced local precipitation or developed into thunderclouds. Figure 12.16 Polar air-earth current densities ju+ and ju- measured on August 4, 1978, at Laramie, Wyoming. The mean total airearth current density is jÏ = (5.1 Â± 0.3) pA/m2. Figure 12.17 Mean diurnal variations of the air-earth current density in relative units over the North Atlantic (Gringel et al., 1978) and at the South Pole (Cobb, 1977). The electric field in the lower atmosphere is vertical and directed downward during fair-weather conditions and large-scale atmospheric homogeneity. In the literature of atmospheric electricity, that direction is defined as the direction a positive charge moves in the electric field (e.g., Chalmers, 1967). At globally representative stations, such as ocean or polar stations, the vertical columnar resistance remains nearly constant during fairweather (Dolezalek, 1972). Here the electric-field strength near the ground shows a diurnal variation with universal time similar to that shown for the air-earth current density in Figure 12.17, both reflecting variations of the ionospheric potential. Over the continents consideration must be given to a varying ionization rate in the first few hundred meters caused by the exhalation of radioactive materials from the Earth. This ionization rate depends strongly on different meteorological parameters, such as convection, and therefore the columnar resistance can no longer be regarded as constant. As shown by IsraÃ«l (1973b) the global diurnal variation of the electric field is normally masked by local variations at these stations. If local generators, such as precipitation, convection currents, and blowing snow or dust,
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 178 also become active, the description becomes increasingly complicated and the vertical electric-field strength Ez can vary considerably. Figure 12.18 Typical variations of the vertical electric field near the ground and their relative amplitude distribution during fair weather (0), haze (â), and fog (â¡) (from Fischer, 1977). Figure 12.18 shows typical variations of Ez at a continental mid-latitude station during fair weather and also during haze and fog (Fischer, 1977). The relative occurrence of the field amplitudes is also shown. During fair weather the variation is small with a mean value of E z about 120 V/m with no negative fields. During haze and fog the variations become much larger and even negative values of E z occur indicating the presence of space charges around the station. The higher positive Ez values are mainly caused by a drastic reduction of the atmospheric conductivity due to the attachment of small ions to haze or fog droplets. The highest values of Ez are measured during rain or snow showers and thunderstorms as shown in Figure 12.19 (Fischer, 1977). For both cases the amplitude distribution shows a typical U pattern with mainly high positive or negative field values. The highest field values can reach 5000 V/m at the ground. This seems to be an upper limit because corona discharges build up a spacecharge layer with the appropriate sign so as to reduce the original field values. Examples of anthropogenic influences are shown in Figures 12.20 and 12.21. Figure 12.20 shows the undisturbed and disturbed electricfield values near the ground on the upwind and downwind side of a high-voltage power line. Figure 12.21 shows the undisturbed and disturbed fields values near a large city in Germany (Fischer, 1977). Whereas the station at the south shows almost a fair-weather field pattern, the values at the northern station exhibit large variations, and even negative values of E z occur. The reasons for the large variations at the disturbed stations are in both cases drifting pollution and/or space charges. Above the ground the vertical electric field Ez drops rapidly with increasing altitude owing to the increasing atmospheric conductivity. Figure 12.22 shows the decrease of the vertical electric field with altitude during fair weather, during cloudiness without precipitation, and during haze and fog as measured again over Weissenau, Germany (Fischer, 1977). The positive sign of E z means that the field vector is pointed downward again. The variations of different profiles, shown by the hachured areas, are mainly caused by conductivity variations, especially in the lower troposphere, rather than by variations of the ionospheric potential itself. The scatter is greatly increased during periods of cloudiness and haze or fog, whereas the mean profile of the same 20 balloon flights (indicated by the thick curve in Figure
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 179 12.22) still shows a pattern typical for fair-weather conditions. During rain or snow and especially in thunderclouds, the scatter in the field values becomes much larger, including regions with large negative field values. Temporal variations are fast, and the horizontal components of the electric-field strength can reach the same order of magnitude as the vertical components as measured by Winn et al. (1978). These large variations of the electric-field strength in shower and thunderclouds are caused by regions of high-space-charge density of both signs in these clouds. Figure 12.19 Same as Figure 12.18 for rain or snow showers ( â ) and for thunderstorms (from Fischer, 1977). Figure 12.20 Influence of a high-voltage power line (220 kV) on the electric field near the ground (black station) compared with the electric field at the undisturbed station (Fischer, 1977). Above the tropopause the vertical electric-field strength continues to decrease nearly exponentially as the atmospheric conductivity increases and normally drops to around 300 mV/m at 30 km at mid-latitudes. Holzworth and Mozer (1979) showed that solar proton events can cause large reductions of the stratospheric electric-field values at high latitudes by more than an