The thermosphere is the layer of Earth’s atmosphere directly above the mesosphere. The thermosphere extends from an altitude of about 80 km (50 mi) to 600 km (370 mi). It consists of four sub-layers, which are the upper mesosphere, stratopause, ionosphere, and exosphere. The thermosphere is heated by ultraviolet radiation from the Sun.
The temperature in the thermosphere generally increases with height. The lower part of the thermosphere (up to about 350 km or 220 mi) is sometimes called the “exosphere”, although this term is more often used to refer to the outermost parts of Earth’s atmosphere. The International Space Station orbits within this region, as do many artificial satellites.
The upper part of the thermosphere (above 350 km or 220 mi), where most atmospheric drag occurs, is sometimes called the “upper atmosphere”. Here, molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. This produces very interesting effects such as noctilucent clouds and polar aurorae.
Some definitions extend the top of the thermosphere to be at 1 mPa (10 hPa; 0.1 kbar), making it coincident with what is variously called either a heterosphere or a homopause. Others place its top at 50 mPa. Nevertheless, all published models indicate that pressure indeed steadily declines with altitude in this region up until at least 1 mPa according to scale heights derived from hydrostatic equilibrium considerations.[citation needed]