The Earth atmosphere can be broadly divided into three distinct regimes:
• The homosphere which comprises the regions of the troposphere (surface up to ~ 10 - 12 km altitude), the stratosphere (~ 10 - 12 km up to 50 km), and the mesosphere (~50 km up to about 90 km);
• The thermosphere which extends from about 90 km altitude to approx. 400 km (depending on solar and geomagnetic activity levels);
• The exosphere which extends from the top of the thermosphere and extends into space.
More information on these three regions can be found in Annex G.