part of the Earth’s atmosphere above the thermosphere for which the mean free path exceeds the scale height, and within which there are very few collisions between atoms and molecules
NOTE 1 Near the base of the exosphere atomic oxygen is normally the dominant constituent.
NOTE 2 With increasing altitude, the proportion of atomic hydrogen increases, and hydrogen normally becomes the dominant constituent above about 1 000 km. Under rather special conditions (i.e. winter polar region) He atoms can become the major constituent over a limited altitude range.
NOTE 3 A small fraction of H and He atoms can attain escape velocities within the exosphere.