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 Spatiale, Brussels, Belgium; Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA). Perhaps part of this obscurity arises because aeronomy is a relatively new and evolving field of endeavor, with a history dating back no farther than about 1940. The Chambers dictionary of science and technology provides the following defini tion: "aeronomy (Meteor. ). The branch of science dealing with the atmo sphere of the Earth and the other planets with reference to their chemi cal composition, physical properties, relative motion, and reactions to radiation from outer space" This seems to us an appropriate description, and it is reflected throughout the content of this volume. The study of the aeronomy of the middle atmosphere experienced rapid growth and development during the 1970's and 1980's, particularly due to con cern over the possibility of anthropogenic perturbations to the state of the middle atmosphere and its protective ozone layer. As a result, much has been learned regarding both the natural behavior of the atmosphere and the impact of man's activities upon it. In this book we shall attempt to describe the current state of the art as we see it.

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,

"[...] an interesting and well-written overview of the current status of our knowledge of the composition of the middle atmosphere and the basic radiative, dynamical and photochemical processes which maintain it." (Bulletin American Meteorological Society)

This book is a multi-author treatise on the most outstanding research problems in the field of the aeronomy of the Earth’s atmosphere and ionosphere, encompassing the science covered by Division II of the International Association of Geomagnetism and Aeronomy (IAGA). It contains several review articles and detailed papers by leading scientists in the field. The book is organized in five parts: 1) Mesosphere-Lower Thermosphere Dynamics and Chemistry; 2) Vertical Coupling by Upward Propagating Waves; 3) Ionospheric Electrodynamics and Structuring; 4) Thermosphere- Ionosphere Coupling, Dynamics and Trends and 5) Ionosphere-Thermosphere Disturbances and Modeling. The book consolidates the progress achieved in the field in recent years and it serves as a useful reference for graduate students as well as experienced researchers.

Aeronomy, Part A is an attempt to make a comprehensive exposition of the processes in aeronomy, the study of composition, movement, and thermal balance of the upper regions of the planetary atmosphere. The text covers topics such as atmospheric regions and atmospheric density, the temperature and molecular mass of the atmosphere; the permanent constituents and the molecular and isotonic composition of the homosphere; and the transition from the homosphere to the heterosphere. Also covered are topics such as the temperature gradient and energies in the thermosphere, conditions at 200km and above 250km, solar radiation and ionization, and aeronomic reactions. The book is recommended for scientists, especially natural physicists, who would like to know more about the field of aeronomy and its advances.

This book is intended for graduate students in middle atmosphere dynamics courses and will be useful to all research workers in meteorology, aeronomy, and atmospheric chemistry. Furthermore, many of the basic dynamical and physical processes discussed also have broad applicability in other branches of atmospheric dynamics, and will be of interest to those studying such areas as climate dynamics and planetary atmospheres.