F region

The F region of the ionosphere is home to the F layer of ionization, also called the Appleton layer, after the English physicist Edward Appleton. As with other ionospheric sectors, 'layer' implies a concentration of plasma, while 'region' is the area that contains the said layer. The F region contains ionized gases at a height of around 150–800 km 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 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.

The F region is the region of the ionosphere which is very important for HF radio wave propagation. This F region is very anomalous in nature.

F1 and F2 Layers

The F1 layer is the lower sector of the F layer and exists from about 150 to 220 km 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+. Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the turbopause (at ~100 km). 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.

Usage in Radio Communication

Critical F2 layer frequencies are the ones that will not go through the F2 layer.[2][3]

References

  1. ^ Adrian Weiss, Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer, Milliwatt QRP Books, 2011, pp. 1-16, 1-22 to 1-24.
  2. ^ http://www.spacew.com/www/fof2.html
  3. ^ http://books.google.de/books?id=HYAgSzrAiYsC&pg=RA2-PA237-IA2&lpg=RA2-PA237-IA2&dq=define+:+F2-Layer+Critical+Frequency&source=bl&ots=HOWQgLtY2a&sig=e6EZFdpJ7O2UYRipvSHT7DwuL3A&hl=en&ei=qi3OSr25HdjhsAaZoIH4AQ&sa=X&oi=book_result&ct=result&resnum=9