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 OF THE MIDDLE ATMOSPHERE 183 13 Electrical Structure of the Middle Atmosphere George C. Reid NOAA Aeronomy Laboratory INTRODUCTION Conventional usage divides the atmosphere into layers on the basis of the average temperature profile. The stratosphere is the region of positive vertical temperature gradient extending from the tropopause to a height of about 50 km, and the overlying region of negative temperature gradient is the mesosphere, extending to about 85 km altitude, where the lowest temperatures in the atmosphere are reached. The main heat source in both of these regions is provided by absorption of solar-ultraviolet radiation by ozone. At still greater heights lies the thermosphere, in which absorption of extreme-ultraviolet radiation causes the temperature to increase again with height. This chapter is concerned mainly with the electrical properties of the upper stratosphere and the mesosphere. The troposphere and lower stratosphere were considered in Chapters 11 and 12 (this volume) and the thermosphere is discussed in Chapter14. Both the composition and the temperature play important roles in determining the electrical structure. As noted above, the temperature in the stratosphere rises from typical tropopause values near 200 K to values of about 270 K at the stratopause (Ë50 km), above which the temperature decreases to mesopause values that are seasonally and latitudinally variable, occasionally dropping below 140 K in the high-latitude summer. The principal atmospheric constituents are molecular oxygen and nitrogen, just as in the lower atmosphere; but there are a number of minor constituents that are important from the point of view of the electrical properties. Among these are nitric oxide (NO), which diffuses into the region from sources below and above; atomic oxygen (O) and ozone (O3), which are formed locally by photodissociation of O2; and water vapor, which can be transported from the troposphere as well as being locally produced. The role of aerosols in the atmosphere at heights above 30 km is uncertain and controversial and is an area of active study. The occasional presence of noctilucent clouds at the high- latitude summer mesopause and the more regular existence of a summertime polar scattering layer seen by satellites have certainly shown that aerosols (probably ice crystals) can exist near the top of the region, but the gap between the mesopause and the well-known aerosol layers of the lower stratosphere remains relatively unexplored. In what follows, sources of ionization, the ion chemistry that determines the steady-state ion composition, and the present status of our knowledge of aerosol distribution are discussed. The final two sections discuss the theory and measurement of conductivity and electric fields in the middle atmosphere.