The mesosphere

The mesosphere, which extends from about 50 to 90 km altitude, is an atmospheric region characterized by a negative gradient of solar energy absorption, and thus, temperature. Although the distribution of most minor constituents is dominated by photochemistry, vertical transport does have a pronounced effect on many of them. Hence, we discuss the basic dynamic principles and their application to the important mesopheric motions: acoustic-gravity waves, tides, planetary scale waves, and eddy motions.The most important minor electrically neutral constituents are compounds of oxygen, hydrogen and nitrogen. In fact, the allotropes of oxygen are in many ways the most significant because they screen out solar ultraviolet radiation and provide the principal source of mesospheric heating (absorption of solar UV by O₂ and ozone). We also discuss oxides of nitrogen and hydrogen which strongly influence the balance of odd oxygen (O and O₃). Brief discussions of the chemistry of carbon compounds and of excited species are also included.The chemistry of ionic species present in the mesosphere is very important because it strongly influences the propagationand absorption of radio waves. Because of ion clustering and negative-ion formation, such chemistry is extremely complex, and it is only very recently that we have begun to understand it. The current state of knowledge is discussed in some detail. The principles involved in constructing models for predicting the distribution of minor constituents, both neutral and ionic, are presented.