ИСТИНА

We present results of observations of VLF chorus emissions detected by the THEMIS spacecraft within the source region. Using multicomponent wave data of parallel propagating chorus elements measured at some distance from the magnetic field minimum, we have found that the elements propagating equatorward had slightly but systematically higher frequencies and smaller amplitudes compared with simultaneously observed elements propagating away from the equator. This feature is revealed both in individual events and statistically. The exponential growth rates of the elements propagating in both directions had close or equal values. We propose an explanation of the observed feature on the basis of a spatio-temporal nonlinear evolution of the energetic electron distribution function in the course of VLF wave generation. We present results of numerical simulations based on backward wave oscillator equations that demonstrate similar features, and propose a qualitative explanation of the observed frequency shift. The motion of electrons from the equator is accompanied by a decrease in their parallel velocity due to both, the adiabatic mirror force and nonlinear wave-particle interaction, which causes an increase in the wave frequency of the chorus elements generated by such electrons and propagating equatorward. The elements propagating in the opposite direction, i.e., from the equator, are formed by electrons whose parallel velocities increase due to the adiabatic mirror force. Correspondingly, such electrons generate waves at lower frequencies.