A feedback model of cyclotron interaction between whistler-mode waves and energetic electrons in the magnetosphere

Abstract

Cyclotron resonance is employed in a new model of the generation of narrow-band VLF emissions from the magnetosphere. Streaming energetic resonant electrons are temporarily phase-bunched by whistler-mode waves, causing transverse currents to be impressed on the medium. These currents act like circularly polarized antennas, producing stimulated Doppler-shifted radiation. The interaction takes place in an ‘emission cell’ located at or near the equatorial plane. Inclusion of feedback between the stimulated radiation and the incoming particles leads to a self-consistent description of fields and currents in time and space. Sample calculations are made for a wave frequency of 16.5 kHz and a homogeneous interaction region of 750-km length located on the equator at L = 3. When there is a 1-mγ input wave and a stream density of 2.4 m−3, regular pulsations with a period of 110 msec are produced. Above a threshold stream density of 1.5 m−3, self-sustained oscillations grow exponentially when the system is triggered by a short pulse. Following saturation, a steady state is reached in which the frequency bandwidth is zero. Predictions of the model are in good agreement with experimental observations.