f layer(n = noun.location) appleton layer, f region - the highest region of the ionosphere (from 90 to 600 miles up) which contains the highest concentration of free electrons and is most useful for long-range radio transmission;
is a kind of part, region
is a part of ionosphere

appleton layer, boundary layer, conjunctival layer of bulb, conjunctival layer of eyelids, e layer, f, f clef, f number, f region, germ layer, heaviside layer, horny layer, kennelly-heaviside layer, layer, layer cake, letter f, malpighian layer, ozone layer, penicillin f, pipe layer, tissue layer