The earth's magnetosphere is composed of three major components. The first is the solar wind, a fully ionized hydrogen and helium plasma that originates at the sun and streams outward into the solar system at speeds of several hundred kilometers per second. It is pervaded by a large-scale interplanetary magnetic field (IMF), which plays an important role in the interaction with the second essential component, the magnetosphere, a region in space pervaded by the magnetic field. Within about six earth radii of the earth's center, this field is approximately a dipole field whose alignment is slightly tilted from the earth's spin axis. At larger distances, the earth's magnetic field is decidedly non-dipolar. It interacts with the solar wind and the IMF in such a way as to confine the earth's field to a magnetic cavity called the magnetosphere. Inside this cavity, a third component is present: the earth's ionosphere. As the upper atmosphere above 80 km is partially ionized by solar radiation an ionospheric source of plasma forms which consists mainly of electrons, protons and singly charged helium and atomic oxygen ions [Cow95]. It is this ionospheric plasma component of the magnetosphere system that is observed experimentally with the SuperDARN radars and whose properties are analyzed in this thesis. Certain regions in the ionosphere will be referenced frequently in the discussion that follows: the E-region (or E-layer) located at around 100-170 km and the F-region (or F-layer) at around 170-500 km in height. Each layer has a distinct maximum in the electron density versus height profile; the heights of the maxima are roughly 110 km for the E region and 320 km for the F region. They are not really at fixed values but rather are indicative of the center of regions that are very dynamic. Figure 2.1 shows an example of the electron density profile at latitudes where the Saskatoon SuperDARN radar makes measurements.
Figure 2.1: Electron density profile for local noon and midnight at Rabbit Lake, Sask. as derived from the International Reference Ionosphere (IRI) model