IONOSPHERE. The ionosphere is a REGION of the upper atmosphere, from about 85 km (53 MI) to 600 km (370 mi) altitude, and includes the THERMOSPHERE and parts of the mesosphere and exosphere. It is 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.