Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 167 parts of the columnar resistance, are discussed. In the final section the air-earth current and electric fields in the lower atmosphere are considered. Here the results are interpreted from a global viewpoint with perturbations from local generators. Examples of anthropogenic influences on the electric field near the ground are also presented and discussed briefly. ION PRODUCTION IN THE LOWER ATMOSPHERE The electric structure of the troposphere and lower stratosphere depends strongly on the ion-pair production rate and the physical properties of the ions produced. Cosmic rays are the primary source of ionization in the atmosphere range under consideration. Near the Earth's surface over the continents there is an additional component due to ionization by radioactive materials exhaling from the soil. This radioactive ionization component depends on different meteorological parameters and can exceed the cosmic-ray component by an order of magnitude as discussed in Chapter 11. It decreases rapidly with increasing height, and at 1 km it is already significantly less than the contribution due to cosmic rays (Pierce and Whitson, 1964). The ion-production rate by cosmic rays is shown in Figure 12.1 for different geomagnetic latitudes during the years of solar minimum (1965) and solar maximum (1958) based on balloon measurements by Neher (1961, 1967). The existence of the geomagnetic field gives rise to a pronounced latitude effect. Only at latitudes higher than about 60Â° can the full energy spectrum of the cosmic rays reach the Earth and the depth of penetration be limited only by the increasing atmospheric density for low-energy particles. At high latitudes 100-MeV protons can penetrate to about 30 km height, for example. Moving downward to lower latitudes more and more particles with lower energies are deflected by the geomagnetic field and, therefore, are excluded. The geomagnetic equator itself can only be reached by particles with energies greater than about 15 GeV. The hardening of the cosmic-ray spectrum with decreasing latitude is indicated in Figure 12.1 by the lowering of the height at which the maximum ionization rate occurs. Near the equator this maximum ionization rate is observed around 10 km. Figure 12.1 Profiles of the ionization rate at different latitudes in years of the minimum (1965) and maximum (1958) of the 11-yr solar sunspot cycle (Neher, 1961, 1967). Furthermore, the ionization rate depends strongly on solar activity in a sense that at a particular height the ion- production rate is lower during the sunspot maximum and higher during the sunspot minimum, as illustrated in Figure 12.1. The mechanisms are not fully understood, but it appears that irregularities and enhancements of the interplanetary magnetic field tend to exclude part of the lower-energy cosmic rays from the inner solar system (Barouch and Burlaga, 1975). The effect becomes more pronounced with increasing height and/or increasing geomagnetic latitude. At geomagnetic latitudes around 50Â° the reduction of the ion-production rate during the periods of sunspot maximum is about 30 percent at 20 km and about 50 percent at 30 km. More recently this solar-cycle dependence was confirmed by measurements with open balloonborne ionization chambers by Hofmann and Rosen (1979). Analytical expressions for computing the ionization rates dependent on latitude and solar-cycle period are given by Heaps (1978). Superimposed on the 11-yr solar-cycle variation are so-called Forbush decreases (Forbush, 1954), which are somehow related to solar flares and exhibit a temporary reduction of the incoming cosmic-ray flux for periods of a few hours to a few days or weeks (Duggal and Pomerantz, 1977). On the other hand, solar proton events (SPE) can drastically increase the ion-production rate within the stratosphere and, for high-energy solar protons, sometimes even near the ground. The duration of such SPEs is of the order of hours, and they are normally restricted to high-latitude regions, as discussed in more detail in the following chapters.