Helicon modes in uniform plasmas. I. Low m modes

Abstract

Helicons are whistler modes with azimuthal wave numbers. They arise in bounded gaseous and solid state plasmas, but the present work shows that very similar modes also exist in unbounded uniform plasmas. The antenna properties determine the mode structure. A simple antenna is a magnetic loop with dipole moment aligned either along or across the ambient background magnetic field B0. For such configurations, the wave magnetic field has been measured in space and time in a large and uniform laboratory plasma. The observed wave topology for a dipole along B0 is similar to that of an m = 0 helicon mode. It consists of a sequence of alternating whistler vortices. For a dipole across B0, an m = 1 mode is excited which can be considered as a transverse vortex which rotates around B0. In m = 0 modes, the field lines are confined to each half-wavelength vortex while for m = 1 modes they pass through the entire wave train. A subset of m = 1 field lines forms two nested helices which rotate in space and time like corkscrews. Depending on the type of the antenna, both m=+1 and m = −1 modes can be excited. Helicons in unbounded plasmas also propagate transverse to B0. The transverse and parallel wave numbers are about equal and form oblique phase fronts as in whistler Gendrin modes. By superimposing small amplitude fields of several loop antennas, various antenna combinations have been created. These include rotating field antennas, helical antennas, and directional antennas. The radiation efficiency is quantified by the radiation resistance. Since helicons exist in unbounded laboratory plasmas, they can also arise in space plasmas.

Whistler modes in highly nonuniform magnetic fields. III. Propagation near mirror and cusp fields

“…6 in Ref. 46). As the wave propagates toward the increasing strength of the converging field, the parallel wavelength increases and the propagation angles of the oblique wings become highly oblique.…”

New properties of whistler modes

“…Whistler modes are excited with a magnetic loop antenna (4 cm diameter). When its dipole moment is field aligned, the excited wave has the topology of an m = 0 helicon mode or magnetic vortex [ Urrutia and Stenzel , ]. When the dipole moment is perpendicular to B 0 , the circular loop excites an m =+ 1 helicon mode, i.e., a transverse dipole field which rotates around B 0 .…”

“…Superposition in this manner has been proven appropriate in previous work [ Urrutia et al , ; Stenzel and Urrutia , ]. The wave is launched from a loop whose dipole moment points across the uniform field B 0 , thereby exciting a field topology of an m = 1 helicon mode [ Urrutia and Stenzel , ]. The transverse field components ( B x , B y ) peak on axis while the longitudinal component ( B z ) vanishes on axis.…”

“…It excites a whistler mode with topology like an m = 0 helicon mode which can also be visualized as a vortex field. In contrast to an m = 1 helicon mode, the transverse field components vanish on axis while the parallel field component maximizes on axis [see Urrutia and Stenzel , ]. This results in a linear polarization on axis (Figure b).…”

Whistler waves with angular momentum in space and laboratory plasmas and their counterparts in free space

“…If the loop axis is orthogonal to B 0 the propagating field forms an m = +1 helicon mode. Its topology can be described by a dipole field with axis perpendicular to B 0 but rotating azimuthally as the wave propagates axially [127].…”