From Atmosphere to Space

Stratosphere (10-50km)

Positive temperature gradient due to UV absorption in Ozone O_3 -> no weather, over the clouds

Tropopause between 7 km (poles), 10 km (mid-altidues) and 20 km (equator)

Mesosphere (50-85km)

Negative temperature gradient -> no heating mechanism -> Coldest part of the atmosphere (<143 °C)

Tidal forcing and tropospheric eddies lead to mixing (turbospere)

Meteorites typically break apart here

Last zone before space

Thermosphere (85-600km)

EUV and X-ray dissipation breaks apart atoms and create ions -> Ionosphere

Heating from UV -> warmest part of the atmosphere (>2000 °C^[1])

Space is over 100 km which is the Karman line

Gases are rarified -> mean free path lenghts > 100 km -> Rarified gases

Above 160 km sound cannot propagate

no mixing

EUV and X-ray flux is variable with Solar activity

Thermospheric density and composition is variable

Most of LEO is in the thermosphere

High presence of atomic oxygen -> strong reaction with surfaces

Exosphere (>600km)

Gravitationally bound helium and hydrogen, so thin atoms are on ballistic trajectories -> no gas

Many celestial bodies have exospheres that touch the surface (Moon, Mercury, Ganymede)

Elements in the layers

Ionosphere

Is not a layer, it ranges from upper mesosphere to lower exosphere. Significant free electrons.

Day-night effect:

• D-layer only present during day -> Blocks radio waves
• F-layers combine during night