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Middle-Upper Atmosphere

The atmosphere is a layer of gases retained by the Earth’s gravity surrounding our planet. In atmospheric science the term middle atmosphere refers to stratospheric and mesospheric regions, instead upper atmosphere refers to the thermosphere and exosphere. Often in literature the term middle is not used and the two terms low atmosphere and upper atmosphere describe the regions below and above the troposphere. It is the temperature profile in the atmosphere that distinguishes the various regions (see figure). The tropospheric composition is about 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide and trace of other gases. The troposphere contains also a variable amount of water vapour (1% on average), while in the stratosphere contains few parts per million.

Figure 1 Regions of the Earth’s atmosphere

Middle atmosphere

Above the troposphere which contains the largest part of the total mass of the atmosphere starts the middle (mid) atmosphere with the two regions: the stratosphere and the mesosphere. Depending on the latitude the stratosphere ranges from about 8-15 km to about 50 km. Temperature increases with height due to the presence of a relatively high ozone concentration (few parts per million). The mesosphere extends from about 50 km to about 85 km. In this region the temperature decreases with height.

Upper atmosphere

From about 85 km starts the thermosphere that ends at an altitude of about 640 km. The temperature increases with height. In this region neutral constituents are ionized by solar radiation that contributes to the formation of the ionosphere. The ionosphere in combination with the Earth’s magnetic field influences radio propagation. The exosphere ranges from the thermopause to the outer space. In this region, particles are free and may migrate between the esosphere and the interplanetary space. The magnetosphere and the solar wind have a strong influence on particles motion.

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