ABSTRACT
In uniform media that can propagate linear wave perturbations, the only spatial gradients of the fields are the gradients associated with the phase variation of the wave. The plasmas in the solar–terrestrial environment are not uniform. There are gradients in the pressure, the density, the magnetic field, and the velocity. The chapter discusses the propagation of Magnetohydrodynamic waves in media whose properties only change significantly over distances much larger than a wavelength. It shows that the wave is propagated locally as if in a uniform medium. The case of a transverse Alfven wave is unusual but straightforward. In elementary optics, Huygens’ construction requires that each point on a wavefront be regarded as a point source of elementary waves. Each wave on the wavefront evolves for a certain period of time, after which the surface of constructive interference of all the elementary waves represents the new wavefront.
