Introduction

In the previous chapter, we investigated wave propagation through homogeneous plasmas. In this chapter, we shall broaden our approach to deal with the far more interesting case of wave propagation through inhomogeneous plasmas. To be more exact, we shall consider wave propagation in the limit in which the characteristic variation lengthscale, $L$, of equilibrium quantities in the plasma is much longer than the wavelength of the wave. In other words, $k\,L\gg 1$, where $k$ is the wavenumber. In this limit, we expect our wave solutions to closely resemble those found in the previous chapter (recall that the latter solutions correspond to $k\,L\rightarrow \infty$). For the sake of simplicity, we shall (mostly) restrict our investigation to waves propagating through unmagnetized plasmas. However, the techniques described in this chapter can be generalized, in a fairly straightforward manner, to deal with other types of plasma wave (Budden 1985).