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ELECTRICAL STRUCTURE FROM 0 TO 30 KILOMETERS 166 12 Electrical Structure from 0 to 30 Kilometers Wolfgang Gringel UniversitÃ¤t TÃ¼bingen James M. Rosen and David J. Hofmann University of Wyoming INTRODUCTION This chapter deals with the electrical structure of the lower atmosphere, i.e., the troposphere and the portion of the stratosphere below about 30 km. Here the principal observing platforms (not including surface measurements) are balloons. Their limited height range, rather than other physical considerations, is the main reason that the electrical structure above 30 km will be discussed separately in the following chapter. For better understanding of the electrical phenomena taking place in the lower atmosphere and the coupling between them, the concept of a "global circuit" will be briefly touched onâa complete discussion is presented by Roble and Tzur (Chapter 15, this volume). The discovery of the atmospheric conductivity raised a question concerning the origin of the electric fields and the electric currents that were known to exist and flow continuously in the atmosphere. According to the classical picture of the global circuit (Dolezalek, 1972), the total effect of all thunderstorms acting at the same time can be regarded as the global generator, which charges the ionosphere to several hundred kilovolts with respect to the Earth's surface. This potential difference drives the air-earth current downward from the ionosphere to the ground in the nonthunderstorm areas through the conductive atmosphere. The value of this air-earth current density varies according to the ionospheric potential and the total columnar resistance between ionosphere and ground. Finally the local atmospheric electric field must be consistent with this current flowing through a resistive medium, i.e., the atmosphere. In addition to the global generator there also exist effective local generators such as precipitation, convection currents (charges moved by other than electrical forces), and blowing snow or dust. The latter create their own local current circuits and electric fields superimposed on parts of the global circuit. Generators can be regarded as local generators (Dolezalek, 1972) if the resistance from the upper terminal to the ionosphere is much greater than the resistance from that point to the Earth's surface along the shortest possible path and with the consequence that almost no current flows to the ionosphere from this generator. In the following sections we discuss initially the sources of ionization in the lower atmosphere together with solar- induced and latitudinal variations. In the next section a brief review of aerosol distributions in the troposphere and lower stratosphere is presented. Variations following major volcanic eruptions are emphasized. Atmospheric conductivity, small ion concentrations, and ion-mobility measurements are the subject of the third section. Here the influence that solar activity or aerosols have on the conductivity, and therefore on