F region

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 F ₁ layer, and the F ₂ layer. The F ₁ 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 F ₂ layer. The F ₂ 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 F ₁ layer. It is the main reflecting layer for high frequency communications, both during sunlight hours and during the night.

Also called F layer

See also ionosphere