The F region of the ionosphere is home to the F layer of ionization, also called the Appleton layer, after the English physicist Edward Appleton. As with other ionospheric sectors, 'layer' implies a concentration of plasma, while 'region' is the area that contains the said layer. The F region contains ionized gases at a height of around 150–800 km 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 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.
The F region is the region of the ionosphere which is very important for HF radio wave propagation. This F region is very anomalous in nature.
F1 and F2 Layers
The F1 layer is the lower sector of the F layer and exists from about 150 to 220 km 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+. Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the turbopause (at ~100 km). 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 is the F1 layer. However, this depends upon the frequency of a propagating signal. The E layer electron density and resultant MUF 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 (<6 degrees or so) can reach distances of 3000 km via E region refractions.
- The F2 layer exists from about 220 to 800 km 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. 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
- Adrian Weiss, Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer, Milliwatt QRP Books, 2011, pp. 1-16, 1-22 to 1-24.