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On the basis of the ideal MHD equations expressed in terms of Elsässer variables, a new set of equations has been derived that governs the dynamics of the inhomogeneous background plasma and the superimposed incompressible fluctuations. From these equations the dynamic equation for the two-point and two-time correlation tensor has been obtained, and subsequently the equations of motion for the various spectral densities related to energy, cross-helicity and residual energy or the Alfvén ratio have been established. This set of equations offers a new possibility of discussing and perhaps better understanding the mostly incompressible fluctuations observed in the solar-wind plasma and of analysing their radial evolution into interplanetary space and their spectral development. The scope of the paper is limited to giving mainly formal developments of the equations. A detailed evaluation of the many terms in the light of interplanetary observations is intended for the future, but is not presented in this paper.
Marsch et al. (Thu,) studied this question.