Magnetosphere Ionosphere Atmosphere

Atmospheric Regions

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.

Earth's magnetic field stretched out due to the solar wind

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.

Ionospheric Research

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).

The Earth's Magnetosphere

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.