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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 213 50Â° latitude. Do lightning-intensive tropical thunderstorms deliver more or less current than thunderstorms at higher latitudes, even though the ratio of cloud-to-ground to cloud-to-cloud flashes is smaller? A considerable amount of research is necessary to determine the processes responsible for regulating the current flow from thunderstorms into the global circuit. This research is important for understanding the role of worldwide thunderstorm activity as the generator for the global circuit. Electrical Conductivity, Columnar Resistance, and Global Resistance Galactic cosmic rays are the main source of ionization that maintains the electrical conductivity of the atmosphere from the ground to about 60 km in altitude. Near the ground, however, there is additional ionization due to release of radioactive gases from the soil, and above about 60 km solar ultraviolet radiation becomes important. During geomagnetic storms, ionization due to energetic auroral electron precipitation and to auroral x-ray bremsstrahlung radiation and proton bombardment during solar proton events can all be significant sources for the high-latitude middle atmosphere. The galactic cosmic rays that bombard the Earth's atmosphere are influenced by the Earth's geomagnetic field, which produces a magnetic latitudinal effect in the incoming cosmic-ray flux. The full cosmic-ray spectrum is only capable of reaching the Earth at geomagnetic latitudes higher than about 60Â°. At lower geomagnetic latitudes, the lower-energy particles are successively excluded by the Earth's geomagnetic field, and only particles with energies greater than about 15 GeV reach the equator. The cosmic-ray spectrum thus hardens with a decrease in geomagnetic latitude, with the height of the maximum ion production rate decreasing from about 20 km at high latitudes to about 10 km near the equator. The cosmic-ray ion production rate profile for various geomagnetic latitudes during solar-cycle minimum using the data from Neher (1967) is shown in Figure 15.4. The ionization rate increases with geomagnetic latitude, and both the height of the peak and the slope of the ionization rate above the peak also increase. Near the ground there is about a 20 percent variation between the equatorial region and higher latitudes (IsraÃ«l, 1973). The cosmic-ray ionization undergoes a regular solar-cycle variation with maximum values near solar minimum. At high latitudes the ionization rate may vary by about 50 percent at 15 km and about 75 percent at 20 km. At 20 km the ionization-rate variation through the solar cycle is about 40 percent in mid-latitudes and about 20 percent near the equator. In addition to the solar-cycle variation of galactic cosmic-ray flux, there are shorter-term variations that are associated with some magnetic storms, a 27-day quasi-periodic variation, variations due to solar flares, a diurnal variation, and a variation in the amplitude of the diurnal variation with time. All these variations have been reviewed by Forbush (1966). The diurnal variations are generally small. The solar-flare and magnetic-storm variations of cosmic- ray fluxes are larger (about 2 to 20 percent) and are more important for understanding solar-terrestrial electrical coupling mechanisms (Roble and Hays, 1982). The full impact of all these variations on the electrical conductivity and the properties of the global circuit has not yet been evaluated. Figure 15.4 Cosmic-ray ion-production rate vertical profiles at various geomagnetic latitudes (Neher, 1967; R. Williamson, Stanford University, personal communications, 1981). Over land, the natural radioactivity of the solid ground adds to the cosmic-ray ion-production rate, not by direct radiation from the solid surface but in the release of gaseous intermediaries from rocks and soil on the surface and from soil capillaries. These gaseous intermediaries are then carried upward by vertical mass transfer, and radiation from them can affect the ion-production rate within the first kilometer above the Earth's surface (IsraÃ«l, 1973). Above about 60 km the solar ultraviolet radiation ionization exceeds the ion production owing to galactic cosmic rays. The major source of ionization within the mesosphere is nitric oxide, which can be ionized to NO+ by solar Lyman-alpha radiation at 121.6 nm. Above about 80 km, the EUV and soft x-ray radiation from the Sun produces ionization in the ionospheric E and F re
THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 214 gions. These EUV radiations all have a large day-to-day variability as well as periodic 27-day solar rotation and solar- cycle variations. During solar flares the solar EUV and x-ray radiation can be greatly enhanced, thereby increasing the ionization source and electrical conductivity of the ionosphere. The ionization sources mentioned above all contribute to the electrical conductivity of the atmosphere through the production of positive and negative ions and electrons. The electrical conductivity is governed by the number of positive and negative ions and electrons as well as by their respective mobility. The number densities of various ionic species are controlled by complicated chemical reactions between ions and neutral species. Many of the neutral constituents important for ion chemistry, such as water vapor and NO, are furthermore transported by atmospheric motions, thereby increasing the complexity of determining the global distribution of electrical conductivity. All these various processes are discussed by Reid (Chapter 14, this volume). Above about 80 km, the electrical conductivity is governed by collisions between neutrals, ions, and electrons and by the influence of the geomagnetic field on the mobility of the plasma components. The electron gyrofrequency is greater than the electron-neutral collision frequency above about 80 km, and therefore the electrons are restricted by the geomagnetic-field line. For the ions this occurs above about 140 km. The differential motion of the ions and the electrons in the dynamo region between about 80 and 200 km in altitude gives rise to an anisotropic behavior of the conductivity. The conductivity parallel to the geomagnetic-field lines is not affected by the field, and it increases rapidly with altitude, limited only by collisions between electrons and neutrals and ions. The Pedersen conductivity is parallel to an applied electric field and orthogonal to the magnetic field. It is smaller than the parallel conductivity, and it has a maximum near 140 km, where the ion-neutral collision rate equals the gyrofrequency of ions. The conductivity orthogonal to both an applied electric field and the magnetic field is the Hall conductivity, which maximizes near 105 km. The Pedersen conductivity is carried by electrons below 105 km and by ions above that altitude, whereas the Hall current is mainly due to electrons. A typical profile of electrical conductivity through the daytime atmosphere is shown in Figure 15.5. Near the Earth's surface the electrical conductivity is about 10â14 mho/m. It increases exponentially with altitude, having a scale height of about 6 km until about 60 km, where the effects of free electrons become important and there is an abrupt increase in the conductivity. Figure 15.5 Altitude variation of the electrical conductivity in the Earth's atmosphere and ionosphere from the ground to 200 km. The electrical conductivity within the Earth is shown for comparison (Volland, 1982). Above about 80 km the geomagnetic field introduces anisotropic conductivity components. The parallel conductivity continues to increase with altitude, whereas the Hall and Pedersen conductivities peak near 105 and 140 km, respectively, before decreasing with altitude. The electrical conductivity of the Earth is about 10â3 mho/m, and therefore the Earth's atmosphere can be considered a leaky dielectric sandwiched between two highly conducting regionsâthe Earth's surface and the ionosphere. A calculation of the latitudinal variation of the vertical and horizontal electrical conductivity components in the daytime atmosphere during equinox by Tzur and Roble (1983) is shown in Figure 15.6. The electron densities above about 10 km are calculated using the model of Reid (1976, 1977) and the properties of the neutral atmosphere are specified from the model of Solomon et al. (1982a, 1982b). Below 10 km, the electrical conductivity is represented by the Gish formula, with a latitudinal variation as determined from IsraÃ«l (1973). The electrical conductivity increases abruptly above about 60 km because free electrons are present in the daytime ionosphere, and the effect of the geomagnetic-field line becomes apparent above 80 km. The electrical conductivity parallel to the geomagnetic-field line is larger than either the Hall or the Pedersen conductivity, so the vertical component of the electrical conductivity is small in the equatorial region, where the magnetic-field line is horizontal; and the horizontal component of the electrical conductivity is small in the polar region, where the magnetic-field line is vertical. This calculation for the global distribution of electrical conductivity represents an idealized case considering fair- weather conditions. In reality, the conductivity is