Measurements of the far-field resonance cone for whistler mode waves in a magnetoplasma

Abstract

USING a large volume laboratory magnetoplasma the wave fields and resonance cone of the right hand polarised (whistler mode) wave have been mapped at distances of up to 10 whistler wavelengths from a transmitting antenna. The results are in good agreement with theory and suggest that energy is convected away from the antenna along the surface of the resonance cone. Considerable attention has been paid to the generation of waves by a point source in a magnetoplasma, and theoretical analyses^1–5 have shown that for certain combinations of the antenna operating frequency ω, the electron gyrofrequency ω_c, and the plasma frequency ω_p, the fields become singular along a cone whose axis is parallel to B₀, the steady magnetic field. By examining the refractive index surfaces it can be seen that the group velocity of the wave is zero when the ray vector is parallel to the resonance cone of half angle θ (when the wave normal is at an angle π/2−θ to B₀). We restrict ourselves here to a discussion of the radiation pattern for frequencies ω<ω_c≪ω_p. Waves launched by the antenna then propagate in the right hand polarised whistler mode and the angle of the resonance cone is approximately given by θ=sin⁻¹ (ω/ω_c).