The F region of the ionosphere is home to the F layer of ionization, also called the Appleton–Barnett layer, after the English physicist Edward Appleton and New Zealand physicist and meteorologist Miles Barnett. As with other ionospheric sectors, 'layer' implies a concentration of plasma, while 'region' is the volume that contains the said layer. The F region contains ionized gases at a height of around 150–800 km (100 to 500 miles) above sea level, placing it in the Earth's thermosphere, a hot region in the upper atmosphere, and also in the heterosphere, where chemical composition varies with height. Generally speaking, the F region has the highest concentration of free electrons and ions anywhere in the atmosphere. It may be thought of as comprising two layers, the F1 and F2 layers.

The F-region is located directly above the E region (formerly the Kennelly-Heaviside layer) and below the protonosphere. It acts as a dependable reflector of HF radio signals as it is not affected by atmospheric conditions, although its ionic composition varies with the sunspot cycle. It reflects normal-incident frequencies at or below the critical frequency (approximately 10 MHz) and partially absorbs waves of higher frequency.

F1 and F2 layers

The F1 layer is the lower sector of the F layer and exists from about 150 to 220 km (100 to 140 miles) above the surface of the Earth and only during daylight hours. It is composed of a mixture of molecular ions O₂⁺ and NO⁺, and atomic ions O⁺.^[1] Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the turbopause (at ~90 km, 56 miles). This atomic oxygen provides the O⁺ atomic ions that make up the F2 layer. The F1 layer has approximately 5 × 10⁵ e/cm³ (free electrons per cubic centimeter) at noontime and minimum sunspot activity, and increases to roughly 2 × 10⁶ e/cm³ during maximum sunspot activity. The density falls off to below 10⁴ e/cm³ at night.

• The F₁ layer merges into the F₂ layer at night.
• Though fairly regular in its characteristics, it is not observable everywhere or on all days. The principal reflecting layer during the summer for paths of 2,000 to 3,500 km (1200 to 2200 miles) is the F₁ layer. However, this depends upon the frequency of a propagating signal. The E layer electron density and resultant MUF, maximum usable frequency, during high solar activity periods can refract and thus block signals of up to about 15 MHz from reaching the F1 and F2 regions, with the result that distances are much shorter than possible with refractions from the F1 and F2 regions. But extremely low radiation-angle signals (lower than about 6 degrees) can reach distances of 3000 km (1900 miles) via E region refractions.^[2]
• The F₂ layer exists from about 220 to 800 km (140 to 500 miles) above the surface of the Earth. The F₂ layer is the principal reflecting layer for HF communications during both day and night. The horizon-limited distance for one-hop F₂ propagation is usually around 4,000 km (2500 miles). The F₂ layer has about 10⁶ e/cm³. However, variations are usually large, irregular, and particularly pronounced during magnetic storms. The F layer behaviour is dominated by the complex thermospheric winds.

Usage in radio communication

Critical F₂ layer frequencies are the ones that will not go through the F₂ layer.^[3][4] Under rare atmospheric conditions, F2 propagation can occur, resulting in VHF television and FM radio signals being received over great distances, well beyond the normal 40–100 miles (64–161 km) reception area.