Here we will give a qualitative overview on major activity processes in the solar system. Since our main aim is to concentrate on basic aspects and on theoretical results, a full account of the observational background is outside our present scope. However, in the following sections we will summarize the main observational facts that are relevant for our later discussion. For details the reader is referred to the literature. Note that in the present chapter we will largely refrain from giving physical interpretations. They will be discussed in Part IV using the tools provided in Parts II and III.

Geospace

Magnetospheric activity comprises the major global dynamical phenomena of the Earth's magnetosphere including ionospheric processes. It results from the interaction of the solar wind with the Earth's magnetosphere (Fig. 2.1).

The solar wind is characterized by a fast (supersonic) plasma flow from the Sun into interplanetary space. The magnetosphere is the region above the ionosphere that is dominated by the geomagnetic field. The solar wind compresses the Earth's magnetic field on the day-side and stretches it out to a long tail (magnetotail) on the night-side of the Earth (Fig. 2.1). A bow shock wave stands in front of the magnetosphere, which has a rather thin boundary, the magnetopause. Its thickness, which varies considerably, can become as small as a few hundred km.