On the normal-mode frequency spectrum of kinetic magnetohydrodynamics

This paper presents an explicit proof that, in the kinetic magnetohydrodynamics framework, the squared frequencies of normal-mode perturbations about a static equilibrium are real. This proof is based on a quadratic form for the square-integrable normal-mode eigenfunctions and does not rely on demonstrating operator self-adjointness. The analysis is consistent with the quasineutrality condition without involving any subsidiary constraint to enforce it, and does not require the assumption that all particle orbits be periodic. It applies to Maxwellian equilibria, spatially bounded by either a rigid conducting wall or by a plasma-vacuum interface where the density goes continuously to zero.