Where does the Earth's atmosphere come to an end?

We are all familiar with the inner atmosphere which extends from ground level up to about 100 kilometers. By an altitude of 20 kilometers, we have left behind nearly all of the clouds and weather systems we commonly experience, and are at the boundary of the ozone layer. We have left about 99 percent of our atmosphere behind by the time we reach this altitude. Above the tropopause-ozone layer boundary, we enter what was previously just called the 'stratosphere', but is actually a very complex physical environment. At 50 kilometers, the density of the atmosphere is a million times less than at the surface, and solar ultraviolet and x-rays can penetrate deeply to ionize and heat-up the oxygen and nitrogen atoms there. The inner edge of the ionospheric 'D-layer' is at this altitude, as is, too, the emission from the deepest-penetrating auroral displays. The density of free electrons continues to increase to 1 million electrons/cc by the time you reach 200 kilometers which marks the top edge of the ionospheric F2 layer, and the typical altitude of common auroral displays. At 200 km, the density of the atmosphere itself is 10 billion atoms/cc so it is a long way from the conditions we associate with interplanetary space at 10 - 100 atoms/cc!

The average gas temperature, thanks to solar heating, has risen to 400 K. By the time you arrive at the typical orbit of the Space Shuttle near 300 kilometers, at the inner edge of 'Low Earth Orbit' , the atmospheric density is 1 billion atoms/cc, and when the Space Shuttle moves through this gas at 17,000 miles/hour a glow is produced at the leading edge of the shuttle, so the shuttle is positioned so that it moves tail-first along its orbit. This 'O-glow' is a major problem for experiments in the shuttle bay.

By the time you are 10,000 km from the Earth's surface, you are well-inside the so-called 'geocorona' which is rich in hydrogen, and can be detected by its strong 'Lyman-Alpha' glow. The density has dropped to 1000 atoms/cc and these buzz around with an effective temperature near 15,000 K...hotter than the surface of the sun!

You can explore the atmosphere of the Earth using the MSIS-90 standard atmosphere model.

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