The Sun causes widespread effects on the Earth's environment and most of the phenomena studied by EISCAT have their origin in processes occurring on the solar surface.
In addition to the production of a wide spectrum of electromagnetic radiation - including ultra-violet (uv) and x-rays - the Sun is continuously evaporating to form a super-sonic outflow known as the solar wind. This is at such high temperatures that the individual atoms are broken up into their constituent ions and electrons, a state known as a plasma. A property of this solar plasma is that it carries away the magnetic field at the surface of the Sun, transporting it throughout inter-planetary space and into the vicinity of the Earth.
When the solar wind plasma reaches the Earth, these embedded magnetic fields interact with the Earth's magnetic field, distorting it to form a compression on the day-side, and a very elongated tail on the night-side, away from the Sun.
The region within which the Earth's magnetic field is constrained is called the magnetosphere, and the boundary between this volume and the solar wind is marked by a demarcation region, called the magnetopause.
The solar uv, x-rays and charged particles also ionize the upper parts of the Earth's atmosphere resulting in a region, called the ionosphere, which can be studied by radar methods.
The solar wind varies in response to changing conditions on the Sun. As a result, the distortion of the Earth's magnetic field, the position of the magnetopause and the size of the magnetosphere also vary. Solar conditions alter on different time scales, one of which is an eleven year cycle associated with the appearance of cooler areas on the solar surface called sunspots. The years 1989-91 correspond to maximum of the present cycle.
Since the motion of charged particles in both the interplanetary and magnetospheric plasmas is controlled by the magnetic fields there, the structure and dynamics of the magnetosphere and its boundaries play a crucial role in the penetration of solar wind particles into the Earth's environment. In particular, high- latitude magnetic field lines provide routes for these particles to reach the upper atmosphere, where they interact with the neutral gas to produce both the spectacular visual auroral displays seen at high latitudes and additional ionization at polar and auroral latitudes. Electric fields and currents are also transferred between the magnetosphere and the auroral zone ionosphere.
These regions can be studied by radars which transmit powerful radio waves into the ionosphere, where a small fraction of the energy is scattered back to the radar receiver. This scattered signal contains information describing the ionosphere and upper atmosphere. EISCAT uses this technique in the study of solar-terrestrial physics.