Editing F region

{{short description|Layer in ionosphere}}

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 Victor Appleton|Edward Appleton]] and New Zealand physicist and meteorologist [[Miles Aylmer Fulton Barnett|Miles Barnett]]. As with other ionospheric sectors, 'layer' implies a concentration of [[plasma (physics)|plasma]], while 'region' is the volume that contains the said layer. The F region contains ionized gases at a height of around 150–800&nbsp;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&nbsp;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&nbsp;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]] O<sub>2</sub><sup>+</sup> and NO<sup>+</sup>, and atomic ions O<sup>+</sup>.<ref name="Kamide2007">{{cite book|last1=Kamide|first1=Yohsuke|last2=Chian|first2=Abraham C.-L.|title=Handbook of the solar-terrestrial environment|url=https://archive.org/details/handbooksolarter00kami|url-access=limited|date=2007|publisher=Springer|location=Berlin|isbn=978-3-540-46315-3|page=[https://archive.org/details/handbooksolarter00kami/page/n207 199]}}</ref> Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the [[turbopause]] (at ~90&nbsp;km, 56 miles). This atomic oxygen provides the O<sup>+</sup> atomic ions that make up the F2 layer.
The F1 layer has approximately 5 × 10<sup>5</sup> e/cm<sup>3</sup> (free electrons per cubic centimeter) at noontime and minimum [[sunspot]] activity, and increases to roughly 2 × 10<sup>6</sup> e/cm<sup>3</sup> during maximum sunspot activity. The density falls off to below 10<sup>4</sup> e/cm<sup>3</sup> at night.
* The F<sub>1</sub> layer merges into the F<sub>2</sub> 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&nbsp;km (1,200 to 2,200 miles) is the F<sub>1</sub> 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&nbsp;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&nbsp;degrees) can reach distances of 3,000&nbsp;km (1,900 miles) via E&nbsp;region refractions.<ref>Adrian Weiss, ''Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer'', Milliwatt QRP Books, 2011, pp. 1-16, 1-22 to 1-24.</ref>
* The F<sub>2</sub> layer exists from about 220 to 800&nbsp;km (140 to 500 miles) above the surface of the Earth. The F<sub>2</sub> layer is the principal reflecting layer for HF [[radio communications]] during both day and night. The horizon-limited distance for one-[[hop (telecommunications)|hop]] F<sub>2</sub> [[radio propagation|propagation]] is usually around 4,000&nbsp;km (2,500 miles). The F<sub>2</sub> layer has about 10<sup>6</sup> e/cm<sup>3</sup>. 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 F<sub>2</sub> layer frequencies are the frequencies that will not go through the F<sub>2</sub> layer.<ref name="spacew">{{cite web|url=http://www.spacew.com/www/fof2.html|title=Near-Real-Time F2-Layer Critical Frequency Map|publisher=spacew.com|access-date=2014-12-07|archive-date=2014-06-28|archive-url=https://web.archive.org/web/20140628231212/http://www.spacew.com/www/fof2.html|url-status=dead}}</ref><ref name="google">{{cite book|title=The Electronics of Radio|author=Rutledge, D.|date=1999|publisher=[[Cambridge University Press]]|isbn=9780521646451|url=https://books.google.com/books?id=203BJeFu5qQC|pages=2–237|access-date=2014-12-07}}</ref> Under rare atmospheric conditions, [[F2 propagation]] can occur, resulting in [[very high frequency|VHF]] television and FM radio signals being received over great distances, well beyond the normal {{convert|40|-|100|mi|km|lk=on}} reception area. 

==References==
<references/>

*{{FS1037C}}

[[Category:Ionosphere]]
[[Category:Radio frequency propagation]]

[[ru:Ионосфера#Слой F]]