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,