Triggering of Whistler‐Mode Rising and Falling Tone Emissions in a Homogeneous Magnetic Field

Abstract

We perform a self-consistent one-dimensional electromagnetic particle simulation with a uniform magnetic field and open boundaries. The plasma environment consists of cold isotropic electrons, energetic electrons, and immobile ions. The energetic electrons are initialized with a subtracted-Maxwellian distribution with temperature anisotropy. By oscillating external currents with a constant frequency 0.2 fce, where fce is the electron cyclotron frequency, a whistler-mode wave is injected as a triggering wave from the center of the simulation system, and we investigated the process of interactions between the triggering wave and energetic electrons. We find that both rising-tone and falling-tone emissions are triggered through the formation of an electron hole and an electron hill in the velocity phase space consisting of a parallel velocity and the gyro-phase angle of the perpendicular velocities. The rising-tone emission varies from 0.2 fce to 0.4 fce, while the falling-tone varies from 0.2 fce to 0.15 fce. The generation region of the rising-tone triggered emission starts near the injection point of the triggering wave and moves upstream generating new subpackets. The generation region of the falling-tone triggered emission also moves upstream generating new subpackets. The simultaneous formation of the electron hole and hill is identified by separating small and large wavenumber components corresponding to lower and higher frequencies, respectively, by applying the discrete Fourier transformation to the waveforms in space. Based on the simulation results of the whistler-mode triggered emissions, we conclude that the mechanism of frequency variation of whistler-mode chorus emissions works even in a uniform magnetic field.