Very roughly, it starts about 100 kilometers above the Earth, but there is still part of the Earth's atmosphere even at this altitude. The atmosphere just gets less and less dense, and at these altitudes, the 'scale height' is several hundred kilometers, so that means the continued decrease in the atmosphere slows down significantly. Even at 1000 kilometers, there is a trace of the Earth's 'hydrogen cloud' as its outer atmosphere. It is called the geocorona. The above image was obtained from the NASA/ESA SOHO/SWAN instrument. The NASA IMAGE satellite will also take pictures of the geocorona in real time as part of its study of the space environment. You can view these pictures as they arrive after May, 2000 at the IMAGE/POETRY page. Look for the images from the 'GEO' instrument.
Copyright 1997 Dr. Sten Odenwald
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