ABSTRACT
The summary of MHD theory presented in Chapter 10 was heuristic, and based on some common sense arguments. For some strange reason, such an approach is at times being discredited, so we shall here attempt a more rigorous derivation (Lighthill 1960, van Kampen & Felderhof 1967, Boyd & Sanderson 1969). What we should take into account is the nature of the high electrical conductivity of a plasma, namely, the possibility for the electrons to move freely, or almost freely, through an ion background, which can have dynamics of its own. Complex plasma states with many different types of ions, negative ions, partial ionization, etc. can be seen as generalizations of this case, so to be speciÀc we consider in the following a mixture of an electron and an ion gas, where the ion charge is of the same magnitude as the elementary electron charge but with opposite sign. The obvious example is a fully ionized hydrogen gas.
