Sunspots

Abstract

The existence of sunspots has been known since ancient times (Section 1.1), but it was only at the beginning of this century that they were found to be the sites of very strong magnetic fields, and it was realised that they represent the places where huge magnetic flux tubes burst through the solar surface. A theoretical understanding of sunspots has had to await the development of magnetohydrodynamics; however, even now, there is some controversy about answers to fundamental questions, such as: why is a sunspot cool (Section 8.3), what is its equilibrium structure (Section 8.4), and how is it formed (Section 8.6)? Other topics that are discussed in the present chapter include magnetoconvection (Section 8.1) and the process of magnetic buoyancy (Section 8.2) whereby a flux tube deep within the Sun tends to rise towards the surface because it is lighter than its surroundings. Outside active regions the magnetic flux is not spread out uniformly to a weak field of a few Gauss, but instead it is mainly concentrated at supergranulation boundaries (Section 1.3.2) into intense flux tubes, whose properties are discussed in Section 8.7. As a background to this chapter, the reader is referred to Section 2.9 for a summary of flux tube behaviour and to Sections 1.4.2 and 1.3.2B for an account of the observed properties of sunspots and photo-spheric magnetic fields in general.