Thermosphere, << THUR muh sfihr, >> is the uppermost temperature region of Earth‘s atmosphere. Scientists divide the atmosphere into four regions according to temperature: (1) the troposphere, (2) the stratosphere, (3) the mesosphere, and (4) the thermosphere. The thermosphere begins at the mesopause (the upper boundary of the mesosphere) at an average altitude of about 50 miles (80 kilometers). The thermosphere extends into space.
The air in the thermosphere is extremely thin. Molecules of nitrogen and molecules and atoms of oxygen make up most of the lower parts. Starting at about 125 miles (200 kilometers), oxygen becomes more abundant than nitrogen. Above about 600 miles (1000 kilometers), the thermosphere contains mainly hydrogen and helium.
Air temperatures in the thermosphere steadily rise with altitude because its upper parts absorb more radiation from the sun than its lower parts do. The temperature rises from about -130 °F (-90 °C) at an altitude of about 50 miles (80 kilometers) to more than 2000 °F (1000 °C) at about 200 miles (300 kilometers). Daily and seasonal changes in solar activity cause the temperatures in the thermosphere to vary more than those in any other part of the atmosphere. The high temperatures in the upper thermosphere result from the rapid movement of atoms and molecules there. However, satellites and other spacecraft operate in the thermosphere without becoming hot. They stay cool because the air in the thermosphere is too thin to transfer much heat to objects.
Solar radiation and cosmic rays ionize (electrically charge) atoms in the upper atmosphere. The region of ionized particles, known as the ionosphere, extends through parts of the upper mesosphere and into the thermosphere. Atoms in the outermost region of the thermosphere, called the exosphere, can gain enough speed to escape the atmosphere.
See also Air; Ionosphere; Mesosphere; Stratosphere; Troposphere.
