The F1 layer is the lower sector of the F layer and exists from about 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 ~). 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 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 via E region refractions. • The F2 layer exists from about 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 . 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==