The ionosphere is a thick mantle of partly ionized gas surrounding the earth at heights between 80 and 800 km. It was discovered about fifty years ago, and during the last decade has been studied intensively at observatories all over the world. These studies consist primarily of observing the characteristics of radio waves reflected from the ionosphere. Since waves of different frequencies are reflected at different heights, it is possible to determine how the electron density varies with height, up to the height of maximum density at about 300 km. Radio waves which pass this height, however, continue into space and are not reflected. Consequently very little direct information can be obtained from the ground about the characteristics and behaviour of the ionized region above 300 km.