Propagation of sound on line vortices is superfluids: role of ergoregions

Abstract

We (re)cosider the propagation of small disturbances (sound waves) in the presence of a pinned irrotational vortex in a superfluid with the help of the formalism of acoustic spacetimes. We give closed formulas for the scattering angle for sound rays, formulate the sound-propagation problem in the Hamiltonian form, and discuss the form of boundary conditions at the core of the vortex, where the Hamiltonian has a singular point. The wave equation is simplified to a single ordinary differential equation of Mathieu type. We give an extensive discussion of perturbations localized close to the core, which are similar to what is known as the Kelvin waves. The spectra of modes depend strongly on the type of boundary condition employed close to the vortex core. The existence of the gapless Kelvin mode, which is one of the modes with angular number -1, is usually discussed in the context of unpinned vortices in superfluid helium or rotating Bose-Einstein condensates. We prove that this particular mode is absent if the vortex is pinned, and consequently one must discuss the full family of modes in this case. The question of whether or not the acoustic spacetime admits an ergoregion turns out to have a decisive (qualitative) influence on many aspects of sound-propagation phenomena.