The neutral atomsphere is described by varying altitudes, pressure, composition , density of various gases and temperature. It is divided into various regions as seen by the following figure. 



Figure from - Hargreaves, J.K., The solar-terrestrial environment, Cambridge Atmospheric and Space Science Series 5.


Variation due to Temperature

The nomenclature used to describe different regions is "sphere" and for the upper boundary of the particular region is "pause".

1. Troposphere - This is the lowest layer of the atmosphere in which the temperature decreases with increasing height. The boundary
    of the troposphere is tropopause and it is at an altitude of 12 km.
2. Stratosphere - The next layer is stratosphere where the temperature increases with altitude. This is due to the absorption of ozone.         The maximum temperature increase is at the boundary stratopause at around 50 km.
3. Mesosphere - The next layer is the coldest region and the peak of this is at mesopause at around 80-85 km. The temperature mini-        mium is due to lack of enough ozone at such a high altitude.
4. Thermosphere - This is the hottest region of the atmosphere due to constant source of short solar ultra violet radiation.


Variation due to Composition

1. Homosphere - The region below around 100 km is a homoegeous mixture of various gases.
2. Heterosphere - Above this region the gases are not well mixed and there are more lighter gases due to no gravity. The boundary              between the homosphere and heterosphere is the turbopause and is at 100 km. The heterosphere is divided into two different
    regions due to presence of two major gases-   

•  Heliosphere - This region has helium as the major constituent.
  
•  Protonsphere - This region has hydrogen as the major constituent.

Variation due to Density

1. Ionosphere - ionosphere page
2. Magnetosphere - This region is dominated by the earth's magnetic field.