The reader may be surprised to learn that the word "aeronomy" is not found in many of the standard dictionaries of the English language (for exam­ ple. Webster's International dictionary). Yet the term would appear to exist, as evidenced by the affiliations of the two authors of this volume (Institut d'Aeronomie, Brussels, Belgium; Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA). Perhaps part of this obscu­ rity arises because aeronomy is a relatively new and evolving field of endeavor, with a history dating back no farther than about 1940. The Chambers dic­ tionary of science and technology provides the following definition: "aeronomy (Meteor. ). The branch of science dealing with the atmosphere of the Earth and the other planets with reference to their chemical com­ position, physical properties, relative motion,

1. The Middle Atmosphere and Its Evolution.- 1.1 Introduction.- 1.2 Evolution of the Earth's atmosphere.- 1.3 Possible perturbations.- References.- 2. Chemical Concepts in the Atmosphere.- 2.1 Introduction.- 2.2 Thermodynamic considerations.- 2.3 Elementary chemical kinetics.- 2.3.1 Collision theory of bimolecular reactions.- 2.3.2 Unimolecular reactions.- 2.3.3 Termolecular reactions.- 2.4 Term symbols and their use.- 2.4.1 General.- 2.4.2 Selection rules for electronic radiative processes.- 2.5 Photolysis processes.- 2.6 Excited species in the middle atmosphere.- References and bibliography.- 3. Structure and Dynamics.- 3.1 Introduction.- 3.2 Vertical structure and some observed dynamical characteristics.- 3.3 Fundamental description of atmospheric dynamics.- 3.3.1 The primitive equations.- 3.3.2 The quasi-geostrophic potential voracity equation.- 3.4 Effects of dynamics on chemical species.- 3.5 General circulation models.- 3.6 Dynamics of the stratosphere in two dimensions: a conceptual view.- 3.6.1 Zonal means and eddies.- 3.6.2 Descriptions of the mean meridional stratospheric circulation.- 3.7 The importance of wave transience and dissipation.- 3.8 One-dimensional representations of the atmosphere.- References.- 4. Radiation.- 4.1 Introduction.- 4.2 Solar radiation at the top of the atmosphere.- 4.2.1 The sun as a black body.- 4.2.2 The observed solar spectrum.- 4.3 Attenuation of solar radiation in the atmosphere.- 4.3.1 Absorption.- 4.3.2 Scattering by molecules and aerosol particles.- 4.4 Radiative transfer.- 4.4.1 General equations.- 4.4.2 Solution of the equation of radiative for wavelengths less than 4µm: Multiple scattering.- 4.4.3 Solution of the radiative transfer equation at wavelengths longer than 4µm: Absorption and emission of infrared radiation.- 4.5 Thermal effects of radiation.- 4.5.1 Heating due to absorption of radiation.- 4.5.2 Cooling by radiative emission.- 4.6 Photochemical effects of radiation.- 4.6.1 General.- 4.6.2 Absorption cross sections of the principal atmospheric molecules.- 4.6.3 Numerical calculation of photodissociation coefficients.- References.- 5. Composition and Chemistry.- 5.1 General.- 5.2 Oxygen compounds.- 5.2.1 Pure oxygen chemistry.- 5.2.2 The odd oxygen family and some observations.- 5.3 Carbon compounds.- 5.3.1 Methane.- 5.3.2 Methane oxidation chemistry.- 5.3.3 Some end products of methane oxidation: carbon monoxide and dioxide.- 5.4 Hydrogen compounds.- 5.4.1 General.- 5.4.2 Hydrogen radical chemistry.- 5.4.3 The odd hydrogen family and some observations.- 5.5 Nitrogen compounds.- 5.5.1 Sources of stratospheric nitrogen oxides.- 5.5.2 Chemistry of nitrogen and nitric acid in the stratosphere.- 5.5.3 The odd nitrogen family: lifetimes and observations.- 5.5.4 Chemistry of odd nitrogen in the lower thermosphere.- 5.5.5 The odd nitrogen family in the lower thermosphere and mesosphere.- 5.6 Chlorine compounds.- 5.6.1 General.- 5.6.2 Chlorine chemistry.- 5.6.3 The odd chlorine family: lifetimes and observations.- 5.7 Other halogens.- 5.8 Sulfur compounds and formation of aerosol.- 5.9 Generalized ozone balance.- References.- 6. The Ions.- 6.1 Introduction.- 6.2 Formation of ions in the middle atmosphere.- 6.2.1 The effect of solar radiation.- 6.2.2 Effects of energetic particles.- 6.2.3 Comparison of different ionizing processes.- 6.3 Positive ion chemistry.- 6.3.1 Positive ions in the E-region.- 6.3.2 Positive ions in the D-region.- 6.3.3 Positive ions in the stratosphere.- 6.4 Negative ion chemistry.- 6.4.1 Negative ions in the D-region.- 6.4.2 Negative ions in the stratosphere.- 6.5 Effect of ionic processes on neutral constituents.- 6.6 Radio waves in the lower ionosphere.- References.- 7. Possible Perturbations and Atmospheric Responses.- 7.1 Introduction.- 7.2 The importance of coupling in the study of perturbations.- 7.3 The effect of changes in the solar irradiance.- 7.4 Particle precipitation.- 7.5 Volcanic emissions.- 7.6 Anthropogenic emissions.- 7.6.1 Carbon dioxide.- 7.6.2 Methane.- 7.6.3 Nitrous oxide.- 7.6.4 Aircraft in the troposphere and lower stratosphere.- 7.6.5 The chlorofluorocarbons (CFC's).- 7.6.6 Simultaneous perturbations.- References.- Appendix A. Numerical values of physical constants and other data.- Appendix B. Conversion factors.