F region

TheF regionof theionosphereis home to the F layer of ionization, also called theAppleton–Barnett layer,after the English physicistEdward Appletonand New Zealand physicist and meteorologistMiles Barnett.As with other ionospheric sectors, 'layer' implies a concentration ofplasma,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'sthermosphere,a hot region in the upperatmosphere,and also in theheterosphere,where chemical composition varies with height. Generally speaking, the F region has the highest concentration of freeelectronsandionsanywhere in the atmosphere. It may be thought of as comprising two layers, the F1 and F2 layers.

The F-region is located directly above theE region(formerly the Kennelly-Heaviside layer) and below theprotonosphere.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 thecritical 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 molecularionsO₂⁺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 theturbopause(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 × 10⁵e/cm³(free electrons per cubic centimeter) at noontime and minimumsunspotactivity, and increases to roughly 2 × 10⁶e/cm³during maximum sunspot activity. The density falls off to below 10⁴e/cm³at night.

• The F₁layer merges into the F₂layer at night.
• Though fairly regular in its characteristics, it is not observable everywhere or on all days. The principalreflecting layerduring the summer for paths of 2,000 to 3,500 km (1200 to 2200 miles) is the F₁layer. 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.
• The F₂layer exists from about 220 to 800 km (140 to 500 miles) above the surface of the Earth. The F₂layer is the principal reflecting layer for HFradio communicationsduring both day and night. The horizon-limited distance for one-hopF₂propagationis usually around 4,000 km (2500 miles). The F₂layer has about 10⁶e/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 F₂layer frequencies are the frequencies that will not go through the F₂layer.Under rare atmospheric conditions,F2 propagationcan occur, resulting inVHFtelevision and FM radio signals being received over great distances, well beyond the normal 40–100miles(64–161km) reception area.

References

• This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.