The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere. The thermosphere extends from an altitude of approximately 80 km (50 mi) up to the edge of space at an altitude of 700 km (430 mi).
It extends from approximately 85 kilometers (53 miles) to 640 kilometers (400 miles) above the surface of the planet. The thermosphere is sometimes referred to as the "exosphere", although this term is technically inaccurate since the exosphere is the outermost layer of the atmosphere.
The thermosphere is a region of extremely low density and consists primarily of nitrogen, oxygen, and carbon dioxide molecules. These molecules are so far apart that they can barely interact with one another, and as a result, the thermosphere has very little atmospheric pressure. The thermosphere is heated by ultraviolet radiation from the Sun, which causes the molecules in the atmosphere to become more energetic. This increase in energy makes the thermosphere the hottest layer of the atmosphere, even though it is also the farthest from the Sun.
The thermosphere is also home to the ionosphere, a region where the atmospheric molecules are ionized by the Sun's ultraviolet radiation. The ionosphere extends from approximately 85 kilometers (53 miles) to 1,000 kilometers (620 miles) above the surface of the Earth. The ionosphere is important for radio communications, as it reflects radio waves back to the Earth's surface.
The thermosphere was first discovered in the early 19th century by the British astronomer William Herschel. He observed that the temperature of the atmosphere increased with altitude, and he speculated that this was due to the absorption of ultraviolet radiation from the Sun. However, it was not until the early 20th century that the existence of the thermosphere was confirmed by scientific measurements.
