atmosphere, air - the mass of air surrounding the Earth; "there was great heat as the comet entered the atmosphere"; "it was exposed to the air"

D region, D-layer - the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves

Appleton layer, F layer, F region - the highest region of the ionosphere (from 90 to 600 miles up) that contains the highest concentration of free electrons and is most useful for long-range radio transmission

E layer, E region, Heaviside layer, Kennelly-Heaviside layer - a region of the ionosphere (from 50 to 90 miles up) that reflects radio waves of medium length

region, part - the extended spatial location of something; "the farming regions of France"; "religions in all parts of the world"; "regions of outer space"