As one moves upward in altitude in a planetary atmosphere, several important changes in composition and structure are apparent. Most notably, as a consequence of hydrostatic equilibrium, the gas density decreases, i.e. the air becomes “thinner”. The decrease in density is exponential and governed by a scale height which typically varies in the range of about 5–50 km. Concomitant with this density decrease, the atmosphere becomes increasingly transparent to shorter wavelengths in the solar (or stellar, for exoplanets) spectrum. These shorter wavelengths, typically in the mid, far, and eventually, extreme ultraviolet (MUV, FUV, and EUV respectively), can first dissociate and then at higher altitudes, ionize, various gases in the atmosphere and this alters the composition of the atmosphere. Furthermore, with decreasing density, the frequency of collisions between atmospheric molecules decreases to the point where bulk motions such as turbulence are no longer able to mix the atmosphere. Instead, molecular diffusion becomes the more rapid process and this also leads to a composition change whereby the lighter constituents, typically atomic species such as atomic oxygen, diffuse upwards more rapidly than their heavier counterparts such as O2, N2, or CO2. The region where the atmosphere is well mixed is known as the homosphere; the region where diffusive separation dominates is known as the heterosphere. Although this transition takes place over a range of altitudes, it is common to define some reference boundary altitude known as the homopause to divide the two regimes.
A second transition occurs in the thermal structure. The increased exposure of the atmosphere to energetic UV radiation and the greater dominance of atomic species which are typically inefficient infrared radiators means that the temperature increases markedly with increasing altitude. The altitude regime where the temperature exhibits a large positive temperature gradient is known as the thermosphere. Because that portion of the solar UV spectrum which forms the thermosphere is more variable than the longer wavelengths which heat lower altitudes, thermospheres respond much more strongly to solar variability than atmospheres at lower altitudes. While the thermosphere and heterosphere are closely related and generally overlap in altitude, the physical processes which govern their variability are not precisely identical. In this chapter we will discuss both “spheres”, while lumping the two together under the more general label of “upper atmosphere”.