The Earth's atmosphere is divided into several altitude regions, identified by their temperature structure. These include the troposphere, stratosphere, mesosphere and thermosphere. The region above about 50 km, where the atmosphere is partially ionized, is known as the ionosphere. At high latitudes the ionosphere is strongly influenced by particles and electric fields controlled by the magnetosphere.
Arctic and Antarctic regions are therefore very good places from which to observe the interactions between the solar wind and the magnetosphere, auroral ionosphere and upper atmosphere.
Other phenomena observed in the upper atmosphere, such as waves and tides, originate in the lower thermosphere, mesosphere, stratosphere and troposphere. The high latitude ionosphere is also affected by all these processes as well as by those related to the magnetosphere.
A particularly suitable site was selected for the EISCAT radars, which are located in the auroral zone of Northern Europe to study these processes and their effects on the Earth's environment.
The ionosphere has been studied by radio methods since the early years of this century (and the auroral ionosphere even longer using optical methods). In the late 1950's, it was noted that the development of radar power and sensitivity had reached a point where it was theoretically possible to measure the very weak signals scattered incoherently by free electrons in the ionosphere.
Several research systems were built, and most remain operational today, but all were at relatively low latitudes and could not observe the auroral zones where many of the interactions between the solar wind and the Earth's ionosphand atmosphere take place.
These radars demonstrated the value and flexibility of the approach and there was increasing pressure to locate such a facility at high latitude as soon as possible. The XVI General Assembly of the Union Radio Scientifique International (URSI) held in Ottawa, Canada, in 1969 passed resolutions calling for the construction of second generation instruments in the north American and European auroral zones.
In response to these resolutions, the EISCAT Scientific Association was established in 1975 by the scientific research councils of the Federal Republic of Germany, Finland, France, Norway, Sweden and the United Kingdom (the Associates).