The F region of the ionosphere, also known as the Appleton–Barnett layer after physicists Edward Appleton and Miles Barnett, contains ionized gases between about 150–800 km above sea level within the Earth's thermosphere and heterosphere. It has the highest concentration of free electrons and ions in the atmosphere, forming two layers: F1 and F2. Positioned above the E region and below the protonosphere, the F region reliably reflects HF radio signals up to the critical frequency of about 10 MHz, despite variations caused by the sunspot cycle, making it crucial for radio communication.
F1 and F2 layers
The F1 layer is the lower sector of the F layer and exists from about 150 to 220 km (93 to 137 mi) 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+.1 Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the turbopause (at ~90 km; 56 mi). 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 2,000 to 3,500 km (1,200 to 2,200 mi) 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 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 3,000 km (1,900 mi) via E region refractions.2
- The F2 layer exists from about 220 to 800 km (140 to 500 mi) 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 4,000 km (2,500 mi). 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.34 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 40–100 miles (64–161 km) reception area.
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
References
Kamide, Yohsuke; Chian, Abraham C.-L. (2007). Handbook of the solar-terrestrial environment. Berlin: Springer. p. 199. ISBN 978-3-540-46315-3. 978-3-540-46315-3 ↩
Adrian Weiss, Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer, Milliwatt QRP Books, 2011, pp. 1-16, 1-22 to 1-24. ↩
"Near-Real-Time F2-Layer Critical Frequency Map". spacew.com. Archived from the original on 2014-06-28. Retrieved 2014-12-07. https://web.archive.org/web/20140628231212/http://www.spacew.com/www/fof2.html ↩
Rutledge, D. (1999). The Electronics of Radio. Cambridge University Press. pp. 2–237. ISBN 9780521646451. Retrieved 2014-12-07. 9780521646451 ↩