The next and last layer of the atmosphere is the Thermosphere. This layer begins just above the mesopause at a height of about 80 kilometers. The Thermosphere has no real upper limit, rather, it extends out into space to the magnetosphere, which is not part of the atmosphere. The Thermosphere is so big that it is divided into two additional parts: the Ionosphere and the Exosphere. These two spheres will be discussed later.
If you didn't already figure it out, thermo means hot, so the Thermosphere can also be called the hot sphere. A small animation below shows particles in the Thermosphere moving rapidly to represent the heat present.
The actual temperature in the Thermosphere can reach as high as 2000º C! It is so hot here because nitrogen and oxygen in the atmosphere absorb a lot of radiation from space and convert it to heat. This temperature, however, cannot be measured by a conventional thermometer, it would read below 0º C. Special instruments must be used because temperature is a measure of how fast particles move, the faster the particles, the higher the temperature. There are very few particles in the Thermosphere, however, so not enough particles would strike a thermometer to heat it.
The lower Thermosphere is called the Ionosphere; it extends from 80 to 550 kilometers above the Earth's surface. Nitrogen, oxygen, and other particles in the Ionosphere absorb radiation from the sun and become electrically charged. Electrically charged particles are called ions, hence the name: the Ionosphere. These ions are important to radio because they reflect AM radio waves back to earth allowing for long distance messages to be sent. Unfortunately, when solar flares occur on the sun, ion numbers can increase, causing disturbances in radio signals.
The outer layer of the Thermosphere is called the Exosphere. The Exosphere extends from 550 kilometers to thousands of kilometers into space. It is here that satellites orbit the Earth. These satellites control functions from telephone messages to TV programs and are also important in keeping an eye on the Earth's weather.