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The F region of the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays a ...
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 The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...
, a hot region in the upper
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, and also in the
heterosphere The heterosphere is the layer of an atmosphere where the gases are separated out by molecular diffusion with increasing altitude such that lighter species become more abundant relative to heavier species. The heavier molecules and atoms tend to be p ...
, where chemical composition varies with height. Generally speaking, the F region has the highest concentration of free
electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
and
ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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 E, or e, is the fifth letter and the second vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''e'' (pronounced ); plu ...
(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 An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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 The turbopause, also known as the homopause, marks the altitude in an atmosphere below which turbulent mixing dominates. Mathematically, it is defined as the point where the coefficient of Eddy diffusion is equal to the coefficient of molecular di ...
(at ~100 km, 60 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 Sunspots are solar phenomena, phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibi ...
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 Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in s ...
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 In radio transmission maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere (skywave or "skip" propagation) at a specified time, independent of tran ...
, 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.Adrian Weiss, ''Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer'', Milliwatt QRP Books, 2011, pp. 1-16, 1-22 to 1-24. * 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 Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
during both day and night. The horizon-limited distance for one- hop F2
propagation Propagation can refer to: *Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials *Propaganda, non-objective information used to further an agenda *Reproduction, and other forms ...
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. Under rare atmospheric conditions,
F2 propagation F2 propagation (F2-skip) is the reflection of FM broadcasting signals off the F2 layer of the ionosphere. The phenomenon is rare compared to other forms of propagation (such as sporadic E propagation, or E-skip) but can reflect signals thousands ...
can occur, resulting in VHF television and FM radio signals being received over great distances, well beyond the normal reception area.


References

*{{FS1037C Ionosphere Radio frequency propagation ru:Ионосфера#Слой F