The thermosphere and satellite drag

As the ionosphere is embedded in the neutral atmosphere called thermosphere, it is affected neutral winds. The thermosphere is a layer of Earth's atmosphere that extends from about 90 km to between 500 and 1,000 km above our planet. Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C hotter in the daytime than at night, and roughly 500° C hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C to 2,000° C or higher.

Although the thermosphere is considered part of Earth's atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km. The International Space Station orbits Earth within the thermosphere!

Much of the X-ray and UV radiation from the Sun is absorbed in the thermosphere. When the Sun is very active and emits more high-energy radiation, the thermosphere gets hotter and expands. Because of this, the height of the top of the thermosphere is found at an altitude between 500 km and 1,000 km or higher.

Many satellites orbit within the thermosphere and changes in the density of (the very, very thin) air at orbital altitudes, brought on by heating and expansion of the thermosphere, generates a drag force on satellites. Specific scientific models, as those available by PITHIA-NRF and developed by CNES in France, support engineers to calculate this varying drag into account when calculating orbits, and satellites occasionally need to be boosted higher to offset the effects of the drag force.



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