A theory of VLF emissions

Abstract

The work attempts to give a theoretical explanation of the triggering of VLF emissions by magnetospheric whistler morse pulses. First studied is the behaviour of resonant particles in a whistler wave train in an inhomogeneous medium. It is found that second order resonant particles become stably trapped in the wave. After 1–2 trapping periods such particles dominate the resonant particle distribution function, and produce large currents that are readily estimated. The in phase component of current will produce growth rates about n times the linear value when particles have been trapped for n trapping periods. A detailed analysis shows that the reactive component of current is able to cause a steady change in wave frequency. Next a realistic zero order distribution function is selected, and the growth rates and rates of change of frequency are computed in a magnetospheric whistler pulse in the equatorial zone. These results permit a plausible description of the triggering process. Explanations are offered for a number of detailed features of the process. The theory seems to fit in well with observed numerical observations.