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,