The ionosphere is, in a sense, our planet's final frontier. It is the last wisp of Earth's atmosphere that astronauts leave behind when they enter space. The realm of the ionosphere stretches from 50 to 500 miles above Earth's surface where the atmosphere thins to near-vacuum and exposes itself to the fury of the sun. Solar ultraviolet radiation breaks apart molecules and atoms creating a globe-straddling haze of electrons and ions. (Source: NASA.)

The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the Thermosphere. (Source: WikiPedia.)

The existence of charged particles at heights of 80 km (50 miles) and above, signals the beginning of the ionosphere, a region having the properties of a gas and of a plasma. (Source: High Frequency Active Auroral Research Program.)