Exothermic chemical and ionic reactions are grouped into sources and losses for individual ions and minor neutral species. Thus, each reaction is listed at least twice: for every source there are one or more sinks, and there may be more than one source for some species. (There are a few exceptions involving minor species.) The major neutral species, N2, O2 and O, are end products, following the scheme of Figure 5.4.1. The chemical equations are listed with the excess kinetic energy of the reaction and the rate coefficient that is currently most widely accepted. Reaction rate coefficients are obtained, primarily, from laboratory measurements but a few are derived from atmospheric data and modelling and from theoretical work. Some are still controversial. The numerical values listed in Table A5.1 are derived from numerous references in the research literature and are applicable in the temperature range encountered in the thermosphere.

Formulation of the problem

The density and composition of the thermosphere are not uniform over the globe and variations at a given altitude are a function of solar UV illumination, auroral energy input and the effects of transport from one region to another. Variations in density and composition with latitude and longitude have been observed directly by neutral and ion mass spectrometers carried on board many satellites. Horizontal variations in density and composition are small, however, compared to variations with altitude. The ratio of the total density at the lower boundary of the thermosphere to the density at the upper boundary is about seven orders of magnitude which basically accounts for the large change in composition with altitude throughout the region. Consideration of the latitudinal and longitudinal variability will be deferred to Chapter 8 where we discuss three-dimensional thermospheric and ionospheric dynamics. In this chapter we focus on processes that operate locally or vary primarily with altitude, leading to onedimensional equations. At the lower boundary of the thermosphere, the mesopause, the neutral atmosphere is essentially fully mixed but above this level the composition changes markedly with altitude. Even greater variability prevails in the ion composition.

Introduction

Discussion of the dynamic behaviour of the thermosphere and ionosphere has been left for the last chapter for several reasons. The phenomenology is complex. The global scale wind pattern shows little geographic symmetry, being influenced by the seasonal variability over the globe of the input of solar energy and by geomagnetically controlled ion drift. Since momentum and energy sources vary non-uniformly with altitude, the dynamic response likewise has height-dependent structure. Superimposed on the quasi-steady winds and drifts there are waves that may be generated within the thermosphere or propagate from below. The perturbations may be systematic, such as tidal effects, or impulsive, such as auroral storms. The goal is to understand the large scale or global dynamics as well as the small scale or localized perturbations. The dynamic behaviour of the thermosphere and ionosphere cannot be fully understood without knowledge of the structure and energetics of the region. These properties, presented in the preceding chapters, are therefore incorporated as required.

The system is three-dimensional and time-dependent and there exists a wide range of scale lengths and time constants. Predicting the temporal and spatial morphology of the observable parameters that characterize the thermosphere and ionosphere provides a continuing challenge. As this chapter is being written, a fully self-consistent analysis of the coupled ionosphere and thermosphere is a topic that is at the cutting edge of upper atmosphere research.