Hierarchical structure of noncanonical Hamiltonian systems

TL;DR

This paper introduces a systematic method to embed noncanonical Hamiltonian systems into extended phase spaces using phantom fields, revealing hierarchical structures and topological constraints that aid in understanding bifurcations and instabilities.

Contribution

It develops a novel framework for extending Hamiltonian systems with phantom fields, enabling the representation of non-integrable constraints as new Casimir invariants.

Findings

Hierarchical structure of degenerate Poisson manifolds revealed

Extended phase space captures non-integrable topological constraints

Method applicable to bifurcations and instabilities in Hamiltonian systems

Abstract

Topological constraints play a key role in the self-organizing processes that create structures in macro systems. In fact, if all possible degrees of freedom are actualized on equal footing without constraint, the state of "equipartition" may bear no specific structure. Fluid turbulence is a typical example - while turbulent mixing seems to increase entropy, a variety of sustained vortical structures can emerge. In Hamiltonian formalism, some topological constraints are represented by Casimir invariants (for example, helicities of a fluid or a plasma), and then, the effective phase space is reduced to the Casimir leaves. However, a general constraint is not necessarily integrable, which precludes the existence of an appropriate Casimir invariant; the circulation is an example of such an invariant. In this work, we formulate a systematic method to embed a Hamiltonian system in an extended phase space; we introduce phantom fields and extend the Poisson algebra. A phantom field defines a new Casimir invariant, a cross helicity, which represents a topological constraint that is not integrable in the original phase space. Changing the perspective, a singularity of the extended system may be viewed as a subsystem on which the phantom fields (though they are actual fields, when viewed from the extended system) vanish, i.e., the original system. This hierarchical relation of degenerate Poisson manifolds enables us to see the "interior" of a singularity as a sub Poisson manifold. The theory can be applied to describe bifurcations and instabilities in a wide class of general Hamiltonian systems [Yoshida & Morrison, Fluid Dyn. Res. 46 (2014), 031412].