In this chapter we will address the activity of the Earth's magnetosphere, with emphasis placed on the substorm, which is regarded as the dominant dynamical process of the magnetosphere. What can be expected from our theory-oriented approach? Certainly not deterministic predictions, which are excluded not only by the limitations of the present state of the theory, but also by more fundamental properties such as the chaotic nature of large particle ensembles. Rather, the realistic question is this: Equipped with the tools of Parts II and III, how far do we get? Can the tools help us to make the step from a mere phenomenological picture to a description which allows insights and interpretations in terms of physical processes? Where this goal is not reached, can we at least identify realistic possibilities?

To pursue this line, substantial observational input is required, which means that we abandon the strict theoretical point of view, which was appropriate in Parts II and III to generate a set of tools. (Even there, the selection of problem areas, the choice of parameter regimes or of simplifications were influenced by observations.)

A full discussion of all processes observed to be related to magnetospheric activity is far beyond the present scope. In particular, this applies to the wealth of ionospheric phenomena. The aim is to understand the physics of the large-scale magnetospheric phenomena.