Introduction

Most cosmic fluids are plasmas. As was shown in Chapter 4, the internal motions of the fluid produce magnetic fields, which in turn affect this motion. Magnetic forces are, then, fundamental to the birth, structure, and evolution of most cosmic systems. However, considerable effort was made in the past to explain everything in astrophysics by taking only the gravitational force into account. This approach often yielded inadequate results, and the importance of magnetism as a basic cosmic interaction has now begun to be fully appreciated. In attemping to explain any astrophysical system, systematically ignoring magnetic fields is simply naive.

In this chapter particular attention will be paid to two topics: the Sun and the interstellar medium. A basic introduction to the physics of active galactic nuclei is also included. Observations clearly show that we are dealing with complicated systems. Because of the proximity of the Sun, we are able to appreciate how complex a star can be. There is a large variety of transient phenomena at all observable outer layers in the Sun which are now interpreted as structures in which magnetism plays a leading role. Also, the morphology of galaxies and the complex structures of interstellar clouds may reveal the presence of magnetism. Steady-state phenomena, both in the Sun and in the galactic gas, will also be treated here, taking magnetism into account.

The Sun

In order to determine the general properties of the solar atmosphere plasma, we must determine the characteristic quantities defined in Chapter 4.