Plasma physics and magnetohydrodynamics are enormous subjects which play a central role in many aspects of high energy astrophysics. In this chapter, a simple introduction is provided to a number of recurring topics in the physics of diffuse plasmas. Many more details can be found in the classic text The Physics of Fully Ionised Gases by Spitzer (1962) and the recent authoritative survey by Kulsrud, Plasma Physics for Astrophysics (Kulsrud, 2005). The book The Physics of Plasmas by Fitzpatrick, available on-line, provides a clear introduction to all the topics discussed in this chapter (Fitzpatrick, 2008).

Elementary concepts in plasma physics

The plasma frequency and Debye length

We consider the simplest case of a fully ionised plasma consisting of protons and electrons which have equal number densities np = ne. The electrostatic forces between the electrons and protons are very strong and ensure charge neutrality except on small scales, specifically, on scales less than the Debye length λD. Following Fitzpatrick, suppose a layer of the electrons of thickness x is displaced a distance δx relative to the ions. The net effect is to set up two oppositely charged sheets with surface charge density σ = ene δx and the system forms a parallel plate capacitor with opposite surface charges σ on the plates.