The highest atmospheric zone within the ionosphere, at an altitude of c. 140 to 400 km (c. 90 to 250 mi), containing the F layers.
The highest region of the ionosphere, consisting of free electrons generated by the ionizing effect of solar radiation. This region is subdivided into two layers, designated the F1 layer, and the F2 layer. The F1 layer extends from about 150 km (93 mi) to 240 km (149 mi) above the surface of the Earth and exists only during sunlight hours. At night it merges into the F2 layer. The F2 layer extends from about 240 km (149 mi) to 400 km (248 mi). The density of free electrons in this layer is much higher than it is in the F1 layer. It is the main reflecting layer for high frequency communications, both during sunlight hours and during the night.