Like most fundamental concepts in physics, magnetic reconnection owes its appeal to its ability to unify a wide range of phenomena within a single universal principle. Virtually all plasmas, whether in the laboratory, the solar system, or the most distant reaches of the universe, generate magnetic fields. The existence of these fields in the presence of plasma flows inevitably leads to the process of magnetic reconnection. As we shall discuss in more detail later on, reconnection is essentially a topological restructuring of a magnetic field caused by a change in the connectivity of its field lines. This change allows the release of stored magnetic energy, which in many situations is the dominant source of free energy in a plasma. Of course, many other processes besides reconnection occur in plasmas, but reconnection is probably the most important one for explaining large–scale, dynamic releases of magnetic energy.

Figures 1.1–1.4 illustrate the rich variety of plasma environments where reconnection occurs or is thought to occur. The evidence of reconnection in laboratory fusion machines such as the tokamak [Fig. 1.1 (a)] and the reversed–field pinch [Fig. 1.1 (c)] is so strong that there is no longer any controversy about whether reconnection occurs, but only controversy about the way in which it occurs (§9.1). However, as one considers environments which are further away from the Earth, the evidence for reconnection becomes more circumstantial. Most researchers who study the terrestrial aurorae [Fig. 1.2(a)] believe that they are directly or indirectly the result of reconnection in the Earth's magnetosphere, but the evidence for similar phenomena in other planetary magnetospheres [Fig. 1.2(b)] is much smaller (§10.6).