he upper region of the ionosphere, above approximately 160 km altitude. F region electron densities are highly variable, depending on the local time, solar activity, season, and geomagnetic activity. The F region contains the Fl and F2 layers. The F2 layer is more dense and peaks at altitudes between 200 and 600 km. The Fl layer, which forms at lower altitudes in the daytime, has a smaller peak in electron density.