The tearing instability, which was a dominant topic in the previous chapter, is an important example of magnetic reconnection. In the present chapter we will approach magnetic reconnection from a more general point of view and include steady state and three-dimensional processes.

Introduction

Magnetic reconnection can be regarded as the process that resolves the following dilemma. On sufficiently large length and time scales a plasma behaves approximately as an ideal fluid. The main reason is that in this regime generalized Ohm's law (9.7) reduces to its ideal limit (3.60). As a consequence, the magnetic field is frozen into the plasma motion, which sets severe limitations to the accessible dynamical states (Section 3.8).

Already in the early stages of space exploration it became clear that these limitations cannot be reconciled with observations. In particular, space plasma activity seemed to involve the conversion of large amounts of magnetic energy into kinetic energy of bulk plasma and random particle motion and of high energy particle populations. Such conversion, however, appeared to be strongly inhibited, if not ruled out, by the frozen-in condition.

A particularly clear manifestation of that property is the stabilization of one-dimensional current sheets by the constraint (10.50) that was discussed in Section 10.2.2. As seen there, that constraint excludes the change of the topological structure of the magnetic field.