Quadratic invariants and Hamiltonian structure in coupled gyrostat low-order model hierarchies

Abstract

Coupled gyrostat low-order models (GLOMs) are energy-conserving cores of Galerkin-truncated fluid and geophysical systems, including Rayleigh-Benard convection and vorticity dynamics. A single gyrostat always possesses two quadratic invariants; when gyrostats are coupled, the number and geometry of invariants vary sensitively with model configuration, influencing the effective dimension of the dynamics, nonlinear stability, and statistical equilibria. We provide a systematic theory of this dependence. For sparse nested hierarchies of K gyrostats (M=2K+1 modes, no linear feedback), the number of independent quadratic invariants is exactly (M+1)/2; for general GLOMs with all parameters nonzero, energy is the only guaranteed invariant. The standard algebraic approach to finding invariants does not scale with model size. We show instead that many GLOMs admit a non-canonical Hamiltonian structure, with quadratic invariants recoverable as Casimir functions of an explicitly constructible Poisson matrix. The Hamiltonian structure imposes precise, computationally verifiable constraints on the nonlinear coefficients. For Hamiltonian hierarchies, Casimir gradients project consistently across models of increasing complexity, so that invariants are compatible under restriction to subspaces. The clear geometric interpretation of these models enables consistent application of Hamiltonian dynamics across low-order model hierarchies.