F region
The ionospheric layer with the highest free electron concentration.
The F region of the ionosphere, also known as the Appleton–Barnett layer, is a volume of ionized gases located at heights of approximately 150 to 800 km above sea level, within the Earth's thermosphere and heterosphere. It contains the highest concentration of free electrons and ions in the atmosphere and serves as a dependable reflector of HF radio signals, unaffected by atmospheric conditions.
- Height range
- 150 – 800 km
- Critical frequency
- approximately 10 MHz
- F1 layer daytime height
- 150 to 220 km
- F1 layer electron density noontime minim
- 5 × 10^5 e/cm³
- F1 layer electron density noontime maxim
- 2 × 10^6 e/cm³
- F2 layer electron density
- 10^6 e/cm³
- F2 layer one-hop distance
- around 4,000 km
Lore & Background
The F region is named after the English physicist Edward Appleton and New Zealand physicist Miles Barnett. It sits directly above the E region (formerly the Kennelly–Heaviside layer) and below the protonosphere. The region comprises two layers: the F1 layer (150–220 km, present only during daylight) and the F2 layer (220–800 km, the principal reflecting layer for HF communications day and night). The F1 layer consists of molecular ions O₂⁺ and NO⁺ along with atomic O⁺, while the F2 layer is dominated by O⁺ ions from atomic oxygen, which becomes the dominant constituent above the turbopause at about 90 km. The F1 layer merges into the F2 layer at night. The F2 layer's behavior is dominated by complex thermospheric winds, and its variations are often large and irregular, especially during magnetic storms.
Reader's Guide
The F region is the most significant ionospheric layer for long-distance HF radio communication. It reflects normal-incident frequencies at or below its critical frequency (approximately 10 MHz) and partially absorbs higher frequencies. The F2 layer provides a reliable one-hop propagation distance of about 4,000 km, limited by the horizon. During high solar activity, the E layer can refract signals up to about 15 MHz, blocking them from reaching the F region and resulting in shorter propagation distances, though extremely low radiation-angle signals (below about 6 degrees) can still reach 3,000 km via E region refractions. Under rare atmospheric conditions, F2 propagation can extend into VHF frequencies, allowing television and FM radio signals to be received far beyond their normal 40–100 mile range. The F1 layer serves as the principal reflecting layer during summer for paths of 2,000 to 3,500 km, depending on signal frequency.
Did You Know?
- The F region is also called the Appleton–Barnett layer.
- The F1 layer exists only during daylight hours, from about 150 to 220 km.
- The F2 layer has about 10^6 free electrons per cubic centimeter.
- Under rare conditions, F2 propagation can allow VHF signals to travel well beyond 100 miles.
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