Exact Nonlinear Decomposition of Ideal-MHD Waves Using Eigenenergies II: Fully Analytical EEDM Equations and Pseudo-Advective Energies

Abstract: Physical insight into plasma evolution in the magnetohydrodynamic (MHD) limit can be revealed by decomposing the evolution according to the characteristic modes of the system. In this paper we explore aspects of the eigenenergy decomposition method (EEDM) introduced in an earlier study (Raboonik et al. 2024 , ApJ, 967:80). The EEDM provides an exact decomposition of nonlinear MHD disturbances into their component eigenenergies associated with the slow, Alfv\'en, and fast eigenmodes, together with two zero-frequency eigenmodes. Here we refine the EEDM by presenting globally analytical expressions for the eigenenergies. We also explore the nature of the zero-frequency ``pseudo-advective modes'' in detail. We show that in evolutions with pure advection of magnetic and thermal energy (without propagating waves) a part of the energy is carried by the pseudo advective modes. Exact expressions for the error terms associated with these modes--commonly encountered in numerical simulations--are also introduced. The new EEDM equations provide a robust tool for the exact and unique decomposition of nonlinear disturbances governed by homogeneous quasi-linear partial differential equations, even in the presence of local or global degeneracies.