f region(n = noun.location) appleton layer, f layer - 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

aphotic region, biogeographical region, bluegrass region, cortical region, d region, e region, f, f clef, f layer, f number, french region, geographic region, geographical region, infernal region, italian region, letter f, lothian region, motor region, nether region, penicillin f, photic region, polar region, pubic region, red region, region, sensorimotor region, tidewater region, unknown region, xinjiang uighur autonomous region