Generation of Whistler Waves by Helical Electron Beams

Abstract

SEVERAL investigations¹ published within the past few years deal with the interaction of a stationary plasma in the outer regions of the ionosphere (exosphere) with an assembly of charged particles trapped in the Earth's magnetic field. Such an interaction results in electromagnetic radiation within the audible range which is detected at the Earth's surface in the form of continuous white noise or whistlers. The trapped particles spiral around the lines of the magnetic field B₀ and form a ‘helical beam’, characterized by velocity components β_⊥c and β_∥C which are, respectively, perpendicular and parallel to B₀ (c is the velocity of light). A whistler wave thus produced and aligned in the direction of B₀ is usually described by the equalities: in which ω is the frequency, k is the wave number, β² = β_∥² + β_⊥², Ω_e is the electron gyrofrequency, and n = ω_e²/ω(ω − Ω_e) is the refractive index for the whistler mode (ω_e is the plasma frequency).