Whistler modes in highly nonuniform magnetic fields. II. Propagation in three dimensions

Abstract

In a large laboratory plasma, the properties of whistler modes are investigated in highly nonuniform magnetic fields. In an extension to previous measurements in two dimensions (2D), the present work shows new phenomena such as wave splitting in the third dimension and shedding of cross-field helicon-like modes. Three-dimensional (3D) data also permit the correct calculations of the field derivatives (∇⋅, ∇×), helicity density (J ⋅ B), Hall electric fields, phase and energy flow, and out-of-plane field structures, which are not visible from 2D data. Novel findings are the loss of the angular momentum of an m = 1 helicon mode, the splitting of a single wave packet into two wave packets in the direction of the loop axis, and the shedding of perpendicular whistler modes with angular momentum. The 3D effects cannot be explained by nonuniformities in the density and the 2D ambient magnetic field B0. They may arise from the conservation of orbital angular momentum whose direction changes along a curved magnetic field. It results in a precessional motion which creates asymmetries in the third dimension. Further effects are the interference of oppositely propagating helicon modes on circular field lines which creates linear polarization near the conjugate point of the antenna. Detached whistler modes are excited in the oscillating near-zone field. The waves propagate nearly perpendicular to the ambient field. The field polarization is right-hand circular around the oblique wave vector k but not around B0. Since the wave field is force-free the wave magnetic field lines form twisted field lines or writhed flux tubes. From streamlines of hodogram normals, it is shown that the wave exhibits a helical phase flow similar to helicon modes. These observations show the complexity of whistler modes in nonuniform magnetic fields, even under the simplest conditions of a uniform, unbounded plasma and linear waves. The results may be of interest to other laboratory plasmas and space plasmas in nonuniform magnetic fields. Meaningful comparisons require 3D field data which are rarely available.