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 150–800 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
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 conven ...
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 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 conven ...
O
2+ 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 d ...
(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 × 10
5 e/cm
3 (free electrons per cubic centimeter) at noontime and minimum
sunspot activity, and increases to roughly 2 × 10
6 e/cm
3 during maximum sunspot activity. The density falls off to below 10
4 e/cm
3 at night.
* The F
1 layer merges into the F
2 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 ...
during the summer for paths of 2,000 to 3,500 km (1200 to 2200 miles) is the F
1 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 transm ...
, 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 F
2 layer exists from about 220 to 800 km (140 to 500 miles) above the surface of the Earth. The F
2 layer is the principal reflecting layer for HF
communications during both day and night. The horizon-limited distance for one-
hop F
2 propagation is usually around 4,000 km (2500 miles). The F
2 layer has about 10
6 e/cm
3. 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
2 layer frequencies are the ones that will not go through the F
2 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