Introduction

The recognition that magnetic fields are an essential component not only of solar and stellar activity but also of the structure of galaxies, quasars, and pulsars has focussed considerable theoretical interest on the origin and maintenance of cosmic magnetic fields. Since the length scales associated with many cosmic magnetic fields are very large, the ohmic decay times (see §4.1 and below) are long, and there is no difficulty in explaining the continued existence of primordial or fossil fields, such as the megagauss fields found in magnetic A-type stars; but the changes observed to occur in many cosmic magnetic fields, over periods which may be short compared with the decay time, entail an interaction between the plasma motions and the existing fields which may also maintain these fields against ohmic decay. This has become known as dynamo action, and, in order to understand evolutionary changes occurring in the solar magnetic cycle, it is necessary to probe further into the underlying theory.

Parker (1970) drew attention to the curious asymmetry throughout the universe between electric and magnetic charge on the one hand, and the corresponding fields on the other.