The upper layer of the ionosphere, approximately 120 to 1500 km in altitude.

the highest region of the ionosphere (from 90 to 600 miles up) that contains the highest concentration of free electrons and is most useful for long-range radio transmission

The lowest layer of the Earth's ionosphere. It is between about 160 and 400 kilometers above Earth's surface. Also called the F Layer. more

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.