Differential-geometrical approach to the dynamics of dissipationless incompressible Hall magnetohydrodynamics: II. Geodesic formulation and Riemannian curvature analysis of hydrodynamic and magnetohydrodynamic stabilities

TL;DR

This paper formulates dissipationless incompressible Hall MHD dynamics as geodesic flows on diffeomorphism groups, analyzing stability via Riemannian curvature, revealing conditions for stability and instability related to mode interactions.

Contribution

It introduces a geodesic formulation of Hall MHD dynamics and analyzes stability through Riemannian curvature, providing new insights into mode interactions and turbulence onset.

Findings

Sectional curvatures vary between positive and negative, indicating stability and instability.

Local mode interactions tend to cause dynamical instability and chaos.

Opposite effects of Hall-term parameter on ion cyclotron and whistler modes.

Abstract

In this study, the dynamics of a dissipationless incompressible Hall magnetohydrodynamic (HMHD) medium are formulated as geodesics on a direct product of two volume-preserving diffeomorphism groups. Formulations are given for the geodesic and Jacobi equations based on a linear connection with physically desirable properties, which agrees with the Levi-Civita connection. Derivations of the explicit normal-mode expressions for the Riemannian metric, Levi-Civita connection, and related formulae and equations are also provided using the generalized Els\"asser variables (GEVs). Examinations of the stabilities of the hydrodynamic (HD, $\alpha=0$) and magnetohydrodynamic (MHD, $\alpha\to0$) motions and the $O(\alpha)$ Hall-term effect in terms of the Jacobi equation and the Riemannian sectional curvature tensor are presented, where $\alpha$ represents the Hall-term strength parameter. It is very interesting that the sectional curvatures of the MHD and HMHD systems between two GEV modes were found to take both the positive (stable) and negative (unstable) values, while that of the HD system between two complex helical waves was observed to be negative definite. Moreover, for the MHD case, negative sectional curvatures were found to occur only when mode interaction was "local," i.e., the wavenumber moduli of the main flow (say $p$) and perturbation (say $k$) were close to each other ($k \approx p$). However, in the nonlocal limit ($k \gg p$ or $k \ll p$), the sectional curvatures were always positive. This result leads to the conjecture that the MHD interactions mainly excite wavy or non-growing motions; however, some local interactions cause dynamical instability that leads to chaotic or turbulent plasma motions. Additionally, it was found that the tendencies of the $O(\alpha)$ effects are opposite between the ion cyclotron and whistler modes. Comparison with energy-Casimir method is discussed.