The thermosphere is located above the mesosphere. The boundary between the mesosphere and thermosphere is called the mesopause. The thermosphere reaches from about 85 km above the surface of the Earth to between 500 and 1,000 km above the surface of the Earth.
The size of the thermosphere changes based on activity from the Sun. When the Sun is shining most brightly on the thermosphere, the heat energy makes the thermosphere expand. When the Sun is less active, the thermosphere temperature drops and it shrinks.
The Greek root “therm” means heat. The thermosphere is the hottest layer of the atmosphere. Temperatures can reach 2000° C because the particles in the thermosphere absorb so much of the Sun’s radiation.
However, it doesn’t feel hot in the thermosphere. This is because only particles can hold heat, so while the air molecules in the thermosphere are very hot, there are so few of them that it feels cold - just like in outer space.
In fact, most of the thermosphere is considered a part of outer space. Satellites and the International Space Station orbit the Earth within the thermosphere.
The thermosphere and the highest parts of the mesosphere are part of the ionosphere. The ionosphere is not a layer in the atmosphere, but it is used to describe the parts of the atmosphere that absorb most of the Sun’s radiation. When particles absorb radiation, they become ions. Ions are charged particles that behave differently than non-charged particles.
One effect of the ions that can be seen from Earth are the Northern Lights or Aurora borealis. Often seen near the poles, these are amazing light shows in the night sky. They are caused when radiation from the Sun collides with the gas particles in the thermosphere.