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.