F2 layer

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. 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 × 10 e/cm (free electrons per cubic centimeter) at noontime and minimum sunspot activity, and increases to roughly 2 × 10 e/cm during maximum sunspot activity. The density falls off to below 10 e/cm at night.