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The upper atmosphere is generally considered to be the region of the thermosphere, which is the thin, outer layer of Earth's atmosphere that starts out around 56 miles (90 kilometers) up and stretches all the way up to about 375 miles (604 kilometers). Spacecraft such as the International Space Station (ISS) or US Space Shuttle typically orbit in the upper atmosphere at a range of about 140 miles (225 kilometers). By contrast, commercial aircraft travel in the much lower level stratosphere that extends to a maximum height of 31 miles (50 kilometers) where the Earth's ozone layer exists.
While the air concentration of the Earth's upper atmosphere in the thermosphere region is very thin compared to what people experience on Earth's surface, this atmosphere is also very hot due to the radiation that it receives from the Sun. Estimates for atmospheric gasses in the upper thermosphere put their temperature at as high as 3,600° Fahrenheit (2,000° Celsius). Due to the rarity of atmospheric gasses at this level, however, their heat is not conveyed to objects passing through the region.
A fifth layer of upper atmosphere that merges with the vacuum of space and is often not considered part of the actual atmosphere is the exosphere. The air density of the exosphere is extremely low, and the region extends from about 375 miles (604 kilometers) up to 6,200 miles (9,978 kilometers). The exosphere merges with regions of the Van Allan radiation belt above it, an area of strongly-charged magnetic particles generated and held in place by the Earth's magnetic field. The exosphere is so thin that there is only about one atom of air or hydrogen per cubic centimeter of space in higher regions, and over 50% of such molecules eventually escape into space. The region is used for many low-orbiting satellites that are unaffected by the rarefied gasses.
One of the unique aspects of the upper atmosphere is that it is the home of auroras, such as the Aurora Borealis and Aurora Australis, or Northern Lights and Southern Lights, which are most clearly visible within 10° to 20° latitude of the North or South Poles. The lights are generated by magnetic effects that the Earth generates when it interacts with the solar wind and atmospheric gasses at this level. The colors that the lights display in the upper atmosphere are dependent upon the type of molecules of air that are being affected, with green to brownish-red colors produced by oxygen, blue from ionized nitrogen, and red from nitrogen at a lower energy state.
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