Laboratory Investigation of Nonlinear Properties of Whistler Waves*

There have been a number of interesting in situ and laboratory observations of whistler wave propagation and stimulated emissions over the past few decades. For example, Stenzel [1975] reported on the self-ducting of large amplitude whistler waves in a laboratory plasma. Those experiments showed that by increasing the driving amplitude, the radiation pattern from a small dipole antenna becomes increasingly narrow, ultimately forming a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. In the space environment, observations of artificially stimulated VLF emissions triggered in the magnetosphere by whistler modes from VLF transmitters have been reported by Stiles and Helliwell [1975]. Emission radiation is assumed to come from the transverse currents formed by counterstreaming electrons that have been temporarily phase bunched by the constant frequency triggering signal. \newline These observations have prompted a new NRL Space Physics Simulation Chamber investigation of whistler wave dynamics. The ultimate goals of these experiments are to understand the mechanisms responsible for ducting, self-focusing, and amplification of whistler waves, to investigate nonlinear whistler-plasma interactions, and to study the secondary emission of whistler waves. The initial experiments concentrate on the ducting of whistler waves in pre-existing density depletions and enhancements. Density structures with controllable scale size and depth will be created using methods previously developed for a Space Chamber investigation of the dynamics of magnetospheric boundary layers [Amatucci et al., 2003].

\newline *Work supported by the Office of Naval Research and DARPA. \newline Stenzel, R.L., Phys. Rev. Lett., 35, 574 (1975). \newline Stiles, G. S. and R. A. Helliwell, J. Geophys. Res., 80, 608 (1975). \newline Amatucci, W. E., et al., Phys. Plasmas, 10, 1963 (2003).