Among other things, the Earth's magnetosphere

(Text adapted from http://seldon.eiscat.no/magnetosphere.html.)

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. Another property of plasmas is that in addition to experiencing all of the hydrodynamic forces of a flowing fluid, they also experience electric and magnetic forces. Thus, a planetary magnetic field represents an obstacle to the flowing solar wind.

When the solar wind plasma reaches the Earth, it interacts 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 distorted magnetic field then forms a cavity in the solar wind flow. This is shown below, with the elongated tail going off to the right.

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 11-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. The incoherent scatter radar at Kangerlussuaq, Greenland uses this technique in the study of solar-terrestrial physics. In particular, the magnetic field lines near the day side boundary of the magnetosphere map to the ionosphere in the vicinity of the radar and other instruments at the Sondrestrom Upper Atmospheric Research Facility near Kangerlussuaq, Greenland.

Many of the processes which couple energy and momentum from the solar wind to the magnetosphere, ionosphere, and upper atmosphere manefest themselves in the high latitude ionosphere and may be measured by the various instruments at the Greenland facility and which comprise the SPARC. These include the dynamic behavior of the ionospheric plasma measured by the radar and the optical auroral emissions measured by the various optical instruments, as well as the cosmic radio noise transmission properties of the ionosphere measured by the riometer.

CREW