The upper portion, 80 to 100 kilometers, above the earth that is characterized by a variation in composition. It closely coincides with the ionosphere and the thermosphere.

The Earth atmosphere above 105 km altitude where species-wise concentration profiles establish due to diffusive equilibrium, with N2 dominance below 200 km, O dominance from 200 to 600 km, and He dominance as of 600 km altitude.

The region of the atmosphere above about 85km where the composition of the air varies with height.