The atmospheric shell characterized by a high ion density. Its base is at about 70 or 80 kilometers and it extends to an indefinite height.
The ionosphere is classically subdivided into layers. 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, starting at about 70 to 80 kilometers and merging with the bottom of the E-layer. The lowest clearly defined layer is the E-layer, occurring between 100 and 120 kilometers. The F1-layer and F2-layer occur in the general region between 150 and 300 kilometers, the F2-layer being always present and having the higher electron density. The existence of a G-layer has been suggested, but is questionable. The portions of the ionosphere in which these layers tend to form are known as ionosphere in which these layers tend to form are known as ionospheric regions, as in D-region, E-region, F-region, G-region. Sudden increases in ionization are referred to as sporadic, as in sporadic E or sporadic D. The above assumption that the ionosphere is stratified in the vertical into discrete layers is currently under serious question. Some evidence supports a belief that ion clouds are the basic elements of the ionosphere. Other investigations appear to reveal the ionosphere as a generally ionized region characterized by more or less random fluctuations of electron density.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use