Whistler wave propagation in a large magnetoplasma

Abstract

The propagation of linear whistler waves is investigated in a very large quiescent collisionless discharge plasma in the parameter regime 0.05<ω /ω_cc/2π≃250 MHz, ω_p/ω_c≳10, where ω_c and ω_p are electron cyclotron and plasma frequency, respectively. Wave dispersion, damping, and polarization are determined from interferometer measurements. Group velocities are measured by propagating phase‐coherent wave bursts. Low frequency wave packets disperse into nose whistlers. The phase and amplitude distribution of antenna‐excited whistler waves is mapped along and across the magnetic field. The different directions of group and phase velocities are clearly demonstrated. Highly oblique whistlers are observed. In weakly nonuniform plasmas (n_e/∇_⊥n_e≫λ_∥) ray tracing is performed directly verifying the theoretical picture of ducting in density troughs. Perfectly ducted undamped whistlers are observed in field aligned troughs of width d≃λ_∥. The profound effect of ray divergence from finite size antennas and of wave refraction effects on the axial amplitude decay has been neglected in most previous experiments and incorrectly interpreted as collisional or cyclotron damping.