thermosphere

The thermosphere is the layer of Earth's atmosphere located between the mesosphere and the exosphere. It extends roughly from about 90 kilometers (56 miles) above the surface to between 500 and 1,000 kilometers (310–620 miles), with the upper boundary sometimes referred to as the thermopause. Boundaries vary with solar activity and geomagnetic conditions.

In the thermosphere, temperature increases with altitude due to the absorption of extreme ultraviolet and X-ray solar radiation by atmospheric gases. Temperatures can reach around 2,000°C (3,600°F) or higher, but the air is so rarefied that heat content is extremely low, and the concept of feeling hot is not applicable at these altitudes. The region is dominated by atomic and molecular species such as atomic oxygen, molecular nitrogen, and molecular oxygen, and it is the zone where intense photoionization occurs, forming part of the ionosphere.

A key characteristic of the thermosphere is its interaction with space weather. The absorption of solar radiation and energetic particle input causes practical changes in density and temperature, which in turn affect satellite drag and orbital lifetimes. The thermosphere also hosts auroral activity, where charged particles precipitate along magnetic field lines and excite atmospheric gases.

The thermosphere is fluid and dynamic, with winds that can reach high speeds and significant variation driven by solar flux and geomagnetic activity. It transitions gradually into the exosphere, where molecules escape into space. The study of the thermosphere informs understanding of satellite operations, radio propagation, and upper-atmosphere chemistry.