ABSTRACT

This chapter discusses the concept of group velocity and deduce expressions for it from the dispersion relation of the waves. The concept can be generalized to apply to non-uniform, slowly varying media. Equations for tracing the paths of the wavepackets or rays can be deduced. MHD waves have the special property that the group velocity depends on the direction of propagation but not on the frequency. The chapter utilises the MHD equations to develop expressions for energy density and energy flux in MHD waves and derive differential equations for the conservation of energy by the wave. It shows that the group velocity is the same as the velocity of energy transport resulting from the wave. This wave energy is only a part of the total energy in the system. The chapter describes how wave energy is separately conserved in a stationary medium and also discusses its meaning.