ionosphere

The region of the atmosphere, extending from approximately 30 to 250 miles (50 to 400 km), in which there is appreciable ionization. The ionized layers in the atmosphere reflect radio waves, making long-wave communications possible over the earth’s surface. The ionosphere is classically subdivided into layers (i.e., D, E, F, and G layers), and the portions of the ionosphere where these layers tend to form are called the D, E, F, and G regions, respectively. Each layer, except the D layer, is supposedly characterized by a more or less regular maximum of electron density. The D layer exists only in the daytime. It is not strictly a layer at all, since it does not exhibit a peak of electron or ion density. It starts at about 30 miles (50 km) and merges with the bottom of the E layer (at a height of 60 miles, or 100 km). The lowest clearly defined layer is the E layer, which occurs at heights between 60 and 90 miles (about 100 to 150 km). The F₁ layer and the F₂ layer occur in the general region between 90 and 210 miles (150 and 350 km); the F₂ layer is always present and has the higher electron density. The G layer exists above the F layer.