ABSTRACT
This chapter discusses the anisotropic propagation properties of waves seen by an observer moving relative to the medium. It considers the transmission and reflection of a wave at the boundary between two counterstreaming media and show that the wave may, in such circumstances, be amplified on reflection. This amplification may be lead to the excitation of cavity modes and, hence, field-line resonances. The chapter also discusses in detail the best way to deal with the energy transfer. It also considers the nature of MHD waves observed in a uniform medium that is moving with constant velocity relative to the observer. An equation can be derived for conservation of the energy associated with the wave. The expression used is the wave energy in that frame and omits first- and second-order terms. When the Galilean transformation is applied to find the energy in the frame in which the plasma is moving, first- and second-order terms are introduced.
