the highest region of the ionosphere, extending from a height of about 150 kilometres to about 1000 kilometres. It contains the highest proportion of free electrons and is the most useful region for long-range radio transmission
Also called: Appleton layer. See also ionosphere
F region in American English
the highest atmospheric zone within the ionosphere, at an altitude of c. 140 to 400 km (c. 90 to 250 mi), containing the F layers