A region in the earth’s atmosphere that has a high concentration of ions and free electrons and extends from about 50 km to over 1000 km in altitude. The ionosphere disturbs the propagation of radiowaves through it by reflecting and attenuating them. It does however allow long-distance radio transmission at frequencies up to about 30 megahertz by successive reflections between it and points on the earth’s surface.
The ionosphere consists of several distinct layers or regions that can change in thickness between day and night and also show seasonal and latitude variations. The D-layer, extending from about 60 to 90 km, has a relatively low concentration of electrons and reflects low-frequency radiowaves. The E-layer extends from about 90 to 120 km, has a higher concentration of electrons than the D-layer, and reflects medium-frequency radiowaves.
The highest layers are the F1-layer, which is a daytime feature centred on about 150 km, and the F2-layer, which is centred on about 300 km. These layers, sometimes known collectively as the Appleton layer, contain the highest concentration of free electrons and reflect high-frequency radiowaves. At night the D- and E-layers become relatively inactive since there is no solar radiation to regenerate ion pairs lost by recombination. The F-layers have a lower density and hence a lower recombination rate of ions. The F2-layer can be used for radio transmission at all times. It is thus the most useful region for long-range radio communication. There is a well-marked ionization maximum for the F-layers, the region above the maximum being the topside ionosphere.
Waves with wavelengths between about 6 millimetres and 20 metres lie within the radio window and are not reflected by the ionosphere but pass straight through it. High-frequency television transmissions fall within this band and require communications satellites, usually in geostationary earth orbit, in order to achieve long-distance television links. Radioastronomy is restricted to wavelengths within the radio window.