Exact Nonlinear Decomposition of Ideal-MHD Waves Using Eigenenergies III: Gravity, Generalized Inhomogeneous Quasi-linear PDEs, Mode Conversion, and Numerical Implementation

TL;DR

This paper extends the Eigenenergy Decomposition Method (EEDM) to inhomogeneous ideal-MHD equations with gravity, enabling precise energy tracking, mode conversion detection, and providing guidelines for numerical implementation in complex plasma simulations.

Contribution

It introduces a generalized EEDM for inhomogeneous MHD with gravity and offers detailed application and numerical guidelines for complex PDEs with conserved quantities.

Findings

Extended EEDM to include gravity effects.

Demonstrated eigenenergy use in mode conversion analysis.

Discussed numerical accuracy and implementation challenges.

Abstract

Precise tracking and measurement of the energy carried by the individual magnetohydrodynamic (MHD) modes has important implications and utility in astrophysical and laboratory plasmas. Previously, this was only achievable in limited linear MHD cases in the $\beta \ll 1$ or $\beta \gg 1$ regimes. In a series of papers, of which this is the third, we introduced the Eigenenergy Decomposition Method (EEDM) and derived exact analytical expressions for the modal energy components--called eigenenergies--of nonlinear 3D disturbances governed by the homogeneous ideal-MHD equations. Here, we extend the method to inhomogeneous ideal-MHD by introducing a source term accounting for gravity, and provide detailed guidelines for applying the decomposition scheme to any general inhomogeneous quasi-linear PDEs that possess a globally conserved quantity, beyond the realm of MHD. Furthermore, we show that the eigenenergies can be used to locate and measure nonlinear mode conversions, which is an additional feature of the method. Finally, we provide well-categorized context for the application of the method to simulations and discuss the possible numerical inaccuracies that may inevitably arise due to discretization. This paper provides a more mature description of the method and its interpretation, and is recommended as the starting point for readers unfamiliar with the method.