Ionosphere, layer of the atmosphere extending from roughly 50 mi (80 km) to 250 or 300 mi (400 or 480 km) above the earth. It is composed of ions (atoms or molecules carrying an electric charge). The atmosphere is thin at that point, and radiation from the sun is able to ionize most of the particles of gas that are present. While the ionosphere is a permanent phenomenon, its structure varies according to season, latitude, and solar activity. The ionosphere is important in radio transmission. Longer-wave signals entering it from below are reflected back toward the earth, making it possible to receive such transmissions over long distances. Shorter waves, such as TV and FM waves, are not reflected by the ionosphere. The aurora borealis (northern lights) takes place in the ionosphere.
See also: Atmosphere.