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,
