Nonlinear wave interaction and spin models in the MHD regime

TL;DR

This paper demonstrates the equivalence of single-fluid and two-fluid electron models with spin considerations in the MHD regime, supported by wave interaction analysis and Manley-Rowe relations, with implications for simulation methods.

Contribution

It shows that a single-fluid model with spin-velocity correlations is equivalent to a two-fluid model without these correlations in the MHD regime, validated through wave interaction analysis.

Findings

Models agree completely in wave interaction predictions

Three wave coupling coefficients obey Manley-Rowe relations

Two-fluid model can be integrated into Particle-In-Cell schemes

Abstract

Here we consider the influence on the electron spin in the MHD regime. Recently developed models which include spin-velocity correlations are taken as a starting point. A theoretical argument is presented, suggesting that in the MHD regime a single fluid electron model with spin correlations is equivalent to a model with spin-up and spin-down electrons constituting different fluids, but where the spin-velocity correlations are omitted. Three wave interaction of 2 shear Alfven waves and a compressional Alfven wave is then taken as a model problem to evaluate the asserted equivalence. The theoretical argument turns out to be supported, as the predictions of the two models agree completely. Furthermore, the three wave coupling coefficients obey the Manley-Rowe relations, which give further support to the soundness of the models and the validity of the assumptions made in the derivation. Finally we point out that the proposed two-fluid model can be incorporated in standard Particle-In-Cell schemes with only minor modifications.