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.