The F region contains ionized gases at a height of around 150–800 km (100 to 500 miles) 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 acts as a dependable reflector of HF radio signals as it is not affected by atmospheric conditions, although its ionic composition varies with the sunspot cycle.
[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 miles).
[3][4] 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.