The upper region of the ionosphere, above approximately 100 miles. F region ionization is highly variable, depending upon the local time, solar activity, season, and geomagnetic activity. The F region contains the F1 and F2 layers. The F2 layer, predominantly responsible for long-distance HF radio propagation, is more dense and peaks in ionization at altitudes between 125 and 375 miles. The F1 layer, which forms at lower altitudes in the daytime, usually possesses less ionization. On the night side of the earth, the F1 rises and the F2 falls to form a single region capable of refracting HF radio signals.

The F region of the ionosphere, also called the Appleton layer, is a region containing ionized gases in the Earth’s upper atmosphere, at a height of 150–400 km above the E region (formerly the Kennelly-Heaviside layer). It acts as a dependable reflector of radio signals as it is not affected by atmospheric conditions, although its ionic composition varies with the sunspot cycle.