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ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 180 order of magnitude. The authors could explain their behavior with the greatly enhanced ionization by solar protons, which in turn enhances the stratospheric conductivity and thereby reduces the local electric fields. Figure 12.21 Disturbed electric field near the ground in the neighborhood of a big city (black station). The electric- field pattern at the south is not disturbed and shows typical fair-weather values (Fischer, 1977). CONCLUSION Galactic cosmic rays are the primary source of ionization in the lower atmosphere. They control the bulk atmospheric conductivity parameter, which in turn is linearly related to the small-ion concentration and small-ion mobility. Although the existence of a solar-induced modulation of the ionization rate and of the atmospheric conductivity is evident, the basic physical mechanisms are not fully understood. More measurements of both parameters (if possible simultaneously) are desirable in order to establish cyclic and transient solar modulation effects. The accuracy of direct measurements of small-ion concentrations and mobilities have shown improvements, but the results among different researchers are still contradictory. Almost nothing is known about the mobility distribution of atmospheric ions throughout the troposphere and lower stratosphere. The existence of heavy ions with masses of several hundred amu, as inferred from mobility measurements, has only just recently been established by mass spectrometer measurements. Problems with the sampling procedure employed can not be overlooked. Simultaneous measurements of ion masses, ion concentrations, and ion mobilities, together with conductivity Figure 12.22 The atmospheric electric field versus altitude during fair weather , cloudiness without precipitation , and during haze and fog (â, â¡). The black curves show typical measurements, the white curves show mean profiles and the hachured areas show the scattering of the values (Fischer, 1977).