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OVERVIEW AND RECOMMENDATIONS 6 coupled directly to the meteorological structure of the storm and/or the storm dynamics, but the lack of simultaneous Maxwell current measurements both on the ground and aloft has not allowed the details of this relationship to be determined. Such measurements will also be complicated by the complexity of the meteorological and electrical environment outside the storm. Electrical Structure of the Atmosphere It has been known for over two centuries that the solid and liquid Earth and its atmosphere are almost permanently electrified. The surface has a net negative charge, and there is an equal and opposite positive charge distributed throughout the atmosphere above the surface. The fair-weather electric field is typically 100 to 300 V/m at the surface; there are diurnal, seasonal, and other time variations in this field that are caused by many factors. The atmosphere has a finite conductivity that increases with altitude; this conductivity is maintained primarily by galactic cosmic-ray ionization. Near the Earth's surface, the conductivity is large enough to dissipate any field in just 5 to 40 minutes (depending on the amount of pollution); therefore, the local electric field must be maintained by some almost continuous current source. Ever since the 1920s, thunderstorms have been identified to be the dominant generator in the global circuit. Most cloud-to-ground lightning transfers negative charge to the ground, and the point discharge currents under a storm transfer positive space charge to the atmosphere. In addition, there are precipitation and other forms of convection currents and both linear and nonlinear conduction currents that must be considered when attempting to understand the charge transfer to the earth by a thunderstorm. The electrical structure of a thunderstorm is complex (see Chapter 8), but it is often approximated simply as a vertical electric dipole. The conductivity of the fair-weather atmosphere near the surface is on the order of 10-14 mho/m, and it increases nearly exponentially with altitude to 60 km with a scale Figure 2 Schematic of various electrical processes in the global electrical circuit.
OVERVIEW AND RECOMMENDATIONS 7 height of about 6 km. The main charge carriers below about 60 km are small positive and negative ions that are produced primarily by galactic cosmic rays. Above 60 km, free electrons become more important as charge carriers and their high mobility produce an abrupt increase in conductivity throughout the mesosphere. Above 80 km, the conductivity becomes anisotropic because of the influence of the geomagnetic field, and there are diurnal variations due to solar photoionization processes. Figure 3 Nomenclature of atmospheric regions based on profiles of electrical conductivity (Ï), neutral temperature, and electron number density. The atmospheric region above about 60 km is known as the equalization layer and is usually assumed to be an isopotential surface and the upper conducting boundary of the global circuit. Currents flow upward from the tops of thunderstorms to this layer where they are rapidly distributed throughout the world. Worldwide thunderstorms maintain a potential difference of 200 to 600 k V between the equalization layer and the surfaceâthe Earth-ionosphere potential. This potential difference, in turn, drives a downward conduction current that is on the order of 2 Ã 10-12 A/ m2 in fair-weather regions and constant with altitude. Today, there are still many details that need to be clarified about the role of thunderstorms as the generators in the global circuit (Figure 2). Upward currents have been detected above thunderclouds, but how these currents depend on storm dynamics, stage of development, lightning frequency, precipitation intensity, and cloud height, for example, is still not known. There is a need for further measurements to quantify the relationships between diurnal variations of the ionospheric potential, the electric field or air-earth current, and worldwide estimates of thunderstorm frequency. Many electrical processes interact within the global circuit, and the following subsections will describe selected processes that occur within certain atmospheric regions (Figure 3). It should be recognized that the global circuit includes mutual electrical interactions between all atmospheric regions.