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 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 {{convert|150|–|800|km|mi|abbr=on}} 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 {{convert|150|to|220|km|mi|abbr=on}} 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⁺.<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 ~{{convert|90|km|mi|abbr=on|disp=semicolon}}). 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 {{convert|2,000|to|3,500|km|mi|abbr=on}} 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&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 {{convert|3,000|km|mi|abbr=on}} 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 F₂ layer exists from about {{convert|220|to|800|km|mi|abbr=on}} above the surface of the Earth. The F₂ layer is the principal reflecting layer for HF radio communications during both day and night. The horizon-limited distance for one-hop F₂ propagation is usually around {{convert|4,000|km|mi|abbr=on}}. 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 frequencies that will not go through the F₂ 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 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