Introduction

In the preceding chapters we have summarized the observed properties of coronal loop structures; we now turn to the interpretation of these properties and provide an account of the physics of coronal loops. In this chapter we shall describe the models of individual loops and in the following chapter we shall consider systems of loops in the context of global models of the solar corona and of stellar coronae in general.

As a preliminary, though, we need to define more precisely than before what is meant by a coronal loop. So far we have employed the intuitive definition of a plasma loop as a continuous structure traceable from a point near the photospheric surface along an arc in the corona to another point at which it returns to the surface. Generally the limited spatial resolution offered by the observations precludes us from saying whether a loop is simple, with a more or less uniform curvature, or whether it possesses a more complicated topology with knots or braids. We have also adopted the view that the visible structures of the corona – the plasma loops – act as tracers of an underlying magnetic field structure, although we have seen that there is little direct evidence of the connection. Thus the first question which we must address in this chapter is the relation between the observed loops and magnetic loops. At this point, the meaning of the term loop becomes a matter of some subtlety.