Introduction

In many ways the simplest description of a quiescent plasma state is provided by a steady state model. We dealt with such models in detail in previous chapters, especially emphasizing static states. However, as discussed in Chapter 4, we cannot expect that exact steady states exist in nature, a more realistic picture of quiescence being that of slow temporal evolution. A convenient description of such a system is a temporal sequence of (approximate) steady state configurations. Each member of such a quasisteady sequence describes a snapshot of the system taken at a particular time during its evolution (Chapter 4).

An important aspect of slow evolution is that in a set of cases relevant for space and astrophysics the evolution leads to the formation of thin current sheets (TCS). That formation is spontaneous in the sense that the small length scale associated with thin current sheets is not present in the external driving forces. Thus we are dealing with spontaneous formation of structure.

As it will turn out, thin current sheets seem to play a crucial role in transitions from quiescence to activity (Parker, 1972; Priest, 1981; Parker, 1994). Parker has argued that braiding of coronal magnetic fields caused by foot-point motion leads to the formation of tangential discontinuities, which cause dynamic resistive dissipation even for extremely small dissipation.