What is the X in X-mas?
The region of the Earth's upper atmosphere lying above the mesosphere and extending from a height of approximately 80 km (50 mi) to between 550 and 700 km (341 and 434 mi) above the Earth's surface. In the thermosphere temperatures increase steadily with altitude, reaching as high as 1,727°C (3,140°F) at the highest elevations. Chemical reactions occur much faster here than on the surface of the Earth. See also exosphere, mesosphere, stratosphere, troposphere., See illustration at atmosphere.
region of increasing temperature in Earth's atmosphere that is located above the mesosphere. The base of the thermosphere (the mesopause) is at an altitude of about 80 km (50 miles), whereas its top (the thermopause) is at about 450 km.