Introduction

In most of the universe the magnetic Reynolds number (Rm, §1.2.2) is very much larger than unity and so the magnetic field is attached very effectively to the plasma. It is only in extremely thin regions where the magnetic gradients are typically a million times or more stronger than normal that the magnetic field can slip through the plasma and reconnect. Thus, for example, a field line initially joining a plasma element at A to one at B in Fig 4.1 may be carried towards another oppositely directed field line CD and a narrow region of very strong magnetic gradient (containing an X-type neutral point) may be formed between them. Then the field lines may diffuse, break, and reconnect, so that element A becomes linked instead to element C (Fig. 4.1).

There are several important effects of this local process:

1. (i) The global topology and connectivity of field lines change, affecting the paths of fast particles and heat conduction, since these are directed mainly along field lines;

2. (ii) Magnetic energy is converted to heat, kinetic energy, and fast particle energy;

3. (iii) Large electric currents and electric fields are created, as well as shock waves and filamentation, all of which may help to accelerate fast particles (Chapter 13).

As we discussed in Chapter 1, two questions that many of the early researchers tried to answer are: what is the nature of field-line breaking and reconnection when it takes place in a steady-state manner; and what is the rate at which it occurs - that is, what is the speed with which magnetic field lines can be carried in towards the reconnection site?