INTRODUCTION

The aim of this chapter is to provide a link between observations of magnetic fields in the Sun and other active stars, and the theory that is presented elsewhere in this volume. I shall begin therefore by considering the observational background and the phenomenological picture that emerges from it. Then I shall go on to discuss a hierarchy of idealized dynamo models that help to explain different aspects of these observations. This material has been the subject of several recent reviews (Weiss 1989, Belvedere 1990, Brandenburg & Tuominen 1991, Stix 1991, DeLuca & Gilman 1991, Rosner & Weiss 1992, Schmitt 1993).

This treatment relies heavily on what has become known as the solarstellar connection. Figure 2.1 shows a Hertzsprung-Russell diagram, with the relative luminosity of the stars plotted as a function of their effective surface temperature (or, equivalently, their spectral type). The stars on the main sequence form a one-parameter family, with their positions determined by their masses. Some of the hot stars to the left of the vertical line have strong magnetic fields, which vary only as a consequence of the star's rotation. The fields in these magnetic stars (the Ap stars) are apparently fossil relics and will not be considered here. Stars to the right of the vertical line are sufficiently cool that hydrogen only becomes ionized beneath their visible surfaces; as a result, they have deep convective envelopes. The combination of convection and rotation is associated with magnetic activity in these cool stars. Their behaviour is similar to that found in the Sun, and it is with them that we are concerned.