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Why study the mesosphere? One can wonder about the importance of the research of physical and chemical phenomena in the mesosphere. We don't live there and apparently, there is no process that threatens life on Earth, in contrast to the events in the stratosphere and the "ozone hole". As in many scientific domains, the study of the mesosphere is primarily fundamental research, i.e. studying our environment and trying to understand why certain phenomena occur. But mesospheric research can be applied research as well, i.e. in this case, checking the effects of human activity on the mesosphere and gathering information about the way in which we influence our own environment. Fundamental research 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) and 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. These "northerns lights" and "southern lights" are another research topic because one still badly understands their spread and effects (heating, chemical reactions) The fundamental research is a goal on itself; motivated by the human desire to get to know and to understand our environment. Moreover applied research is impossible without fundamental research. Applied research The mesosphere plays a role in two important atmospheric phenomena: the "ozone hole" in the stratosphere, influenced by particular processes in the mesosphere, and the global climate change, also called the "greenhouse effect", for which the mesosphere gives us the first indications. 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. Concerning the global (worldwide) climate change, long-term measurements show that the mesosphere has cooled during the last fifty years. This decreasing temperature is probably caused by an increase in the amount of carbon dioxide (CO2) due to human activities. Although carbon dioxide heats up the bottom layers of the atmosphere by absorption of infrared radiation, carbon dioxide emits infrared radiation in the mesosphere, of which half escapes the Earth's atmosphere. This loss of energy causes a decreasing temperature. These perturbations in the mesosphere have faster and more severe consequences than those in the bottom layers of the atmosphere, due to the lower air density in the mesosphere. The perturbations can be observed with the naked eye in the form of polar mesospheric clouds. These clouds condense at very high altitudes, around 80 km. Ordinary clouds only appear in the troposphere. Polar mesospheric clouds are probably clouds of ice crystals becoming visible in the skimming light of the setting sun. It is only with extreme low temperatures that the moistness in the rarefied air of the mesosphere can condense into ice crystals. In northern countries these clouds are seen more and more frequently. This is an additional indication for a global change in the thermal condition of our atmosphere. The considerable sensitivity of the mesosphere to external perturbations makes it an outstanding place to test our knowledge of physics and chemistry in the atmosphere and to warn us for future changes in the lower parts of the atmosphere. So either we can explain the evolution of the climate in the mesosphere, and we will have more confidence in the validity of our predictions concerning the influence of human activity on the global climate, or (more likely) we will not be able to fully explain the evolution and we will be obliged to do further research before we can rely on calculated predictions concerning the increase in the carbon dioxide emissions and the related changes in our climate.