ABSTRACT
Stars that are magnetically active owe this activity to a combination of turbulent convection and rotation. In this review we shall focus on stars like the Sun, which lie on the main sequence and are sufficiently cool that hydrogen becomes ionised below their surfaces, resulting in the presence of a deep outer convection zone. Their magnetic fields can be measured directly through the Zeeman broadening of spectral lines, or inferred from proxy evidence. This is provided by coronal X-ray emission, by H and K emission from singly ionised Ca+, by photometric variability (associated with starspots) or by optical and radio flares – all of which are known to be associated with magnetic activity on the Sun (Tayler, 1997). The Sun is unique, however, in that we can observe detailed magnetic structures on its surface, and we have records of its activity extending back through many centuries; its internal structure is also well-established (see Figure 6.1). On the other hand, the Sun is a single star whose large-scale properties evolve extremely slowly. So it is only through exploiting the solar-stellar connection and examining the magnetic properties of other stars that we can understand how magnetic activity depends on such key parameters as rotation (Wilson, 1994; Mestel, 1999; Schrijver & Zwaan, 2000).
