In this final chapter, we introduce a new level of complexity in the physics of astrophysical fluids, i.e. we recognise that where fluids are composed of charged particles, there is the possibility that their behaviour is modified by electromagnetic fields. Such fluids are called plasmas. Here we will be able to do no more than introduce the fundamental principles of magnetohydrodynamics and select a few simple examples to illustrate some important applications in the interstellar medium.

Magnetic fields are important in many astrophysical situations. They may control the dynamics (e.g. in solar loops and flares, see Figure 13.1) or the source of the radiation from small scales (e.g. pulsars) to very large scales (e.g. radio galaxies). A weak and largely disordered magnetic field with a strength of about 5×10−10 tesla permeates the interstellar medium of the Galaxy. In general, observations show that the magnetic and kinetic energy of the interstellar medium are of similar magnitude, suggesting that magnetic and thermal processes may be closely coupled.

The relevance of magnetic fields for the motion of fluids consisting of charged particles arises because a charge moving with velocity u in a magnetic field B experiences a force proportional to u Λ B. Thus the velocity component of the particle which is parallel to the magnetic field is unaffected, but the perpendicular velocity component gives rise to a force which is orthogonal to both the field direction and the perpendicular velocity direction.