Mesosphere, altitude and temperature characteristics
The mesosphere is a layer within the Earth's atmosphere that extends from about 50 kilometres height (above the stratosphere) and about 90 kilometres (below the thermosphere).
Like tropospheric temperature, mesospheric temperature characteristically decreases with increasing height. The top of the mesosphere is the coldest area of the Earth's atmosphere because temperature may locally decrease to as low as 100 K (-173°C).
Meteoroids and spaceships entering the atmosphere
When extraterrestrial objects (meteoroids, space shuttles) enter the atmosphere, they start heating up in the mesosphere. Penetrating the atmosphere at a high speed, they start heating up because they rub against the oxygen molecules in the mesosphere, where atmospheric density is not be sniffed at.
The mesosphere is generally spoken a “transitional area” between space where satellites orbit and the more familiar terrestrial atmosphere.
How to carry out measurements in the mesosphere?
The mesosphere is an area of the atmosphere poorly known and badly understood. It is very difficult to conduct measurements there because the mesosphere is:
- too high for airplanes (maximal height about 25 km)
- too high for balloons (maximal height about 45 km)
- too low for satellites (minimal height about 130 km)
Mesosphere, a crossing zone between stratosphere and thermosphere
The "classical" atmosphere we live in contains 78% nitrogen and 21% oxygen; all other chemical species form the remaining 1%. Winds are movements of air that move all molecules, independently of their chemical composition, in a similar way. They are the main transport mechanism in the troposphere and the stratosphere.
The region above the mesosphere is the thermosphere, where artificial satellites orbit the Earth. Here, air is extremely rarefied and the composition is rather variable depending on time and place. The main transport phenomenon there is "molecular diffusion", which influences the distribution of the species differently according to their molecular mass.
As a consequence of the very small density of air, the differences in temperature between day and night are considerable. A large part of the gas molecules are ionised; they carry an electric charge and are therefore governed by physical laws that differ totally from the laws governing neutral gasses.
In many ways the mesosphere is a "crossing zone" between these two completely different areas. The physical and chemical processes are progressively sliding from one regime into the other, leading to complex interactions between:
- dynamical phenomena (wind, turbulence, molecular diffusion)
- photochemistry (ozone, nitrogen oxides)
- heating (absorption of ultraviolet light, radiation of infrared light)
These interactions are about as complex as those in the troposphere, in which we live, and we're still far away from a complete understanding of these interactions.
The magnetosphere turns away the charged particles emitted by the Sun, and prevents them from entering the lower layers of the atmosphere, except for the polar regions where collisions between these particles and the neutral air molecules in the mesosphere cause the polar aurorae.
Mesosphere and ozone
The size of the "ozone hole" depends largely on the circulation of air around the poles. In the whole stratosphere winds are driven by dispersion of the gravitation waves in the mesosphere. These waves are vertical oscillations of air masses, which arise in the troposphere as a consequence of the winds above mountain ranges and of storms.
These waves then move upward, like water waves moving in the sea. And like water waves breaking on the beach, gravitation waves end up in the mesosphere because the air density is too weak to keep on transmitting the waves. With this breaking, strong winds driving the total air circulation in the stratosphere arise. One of the necessary conditions for a complete prediction of the evolution of the "ozone hole" is a complete understanding of the complex dynamic phenomena in the mesosphere. This understanding is very difficult because these gravity waves are very variable in time.