Enhanced growth of whistlers due to bunching of untrapped electrons

Abstract

Man‐made signals propagating in the whistler mode in the magnetosphere have been observed to be amplified and to trigger other VLF emissions in the absence of other detectable magnetospheric signals in ground‐based recordings. The absence of other magnetospheric signals implies that the growth rate of the triggered wave is enhanced over background noise. It is shown that, for at least low amplitude triggering waves, the phase bunching of untrapped electrons can account for the observed enhanced growth. The phase bunching is initially produced by perturbations in the electron trajectories due to their interactions with the front of the wave. Due to the inhomogeneity of the geomagnetic field, the phase‐bunched electrons are able to gyroresonantly interact with a later portion of the triggering wave. Because the electrons are phase bunched, the electrons can lose energy to the wave faster than phase‐random electrons and are thereby able to produce enhanced growth. Plasma conditions for which this phase bunching is effective in producing enhanced growth are derived.

Phase‐bunching effects in triggered VLF emissions: Antenna effect

“…On the other hand, it can amplify the secondary wave due to coherent processes. The latter effect requires a self‐consistent approach, and it was considered analytically by Sudan and Ott [1971]; Istomin et al [1976]; Winglee [1985]; Carlson et al [1990]; Trakhtengerts et al [1999] and Demekhov et al [2000]. Below we consider the antenna effect which determines the starting wave amplitude for amplification.…”

“…The effects of phase bunching in triggered VLF emissions were considered in many papers [ Sudan and Ott , 1971; Chu et al , 1980; Matsumoto and Omura , 1981; Sprangle and Smith , 1980; Winglee , 1985]. It was shown that the initial phase bunching of an electron beam can result in enhanced growth rate of a secondary emission.…”

Propagation and absorption of electron-cyclotron maser radiation during solar flares

“…In one case, that component is strongly associated with a trailing wave packet that grows as it propagates through the plasma. The proposal is made that these re-emission events are examples of triggered emission as described by Winglee (1985a) and others, and that re-emission occurs at a frequently slightly above the local harmonic of Oe.…”

“…The re-emission responsible for the growth of pulse c can occur at a frequency above the value of 2f2 e associated with the ambient magnetic field via a process known as triggered emission, which has been studied in association with whistler waves in the magnetosphere (Helliwell, 1967;Winglee, 1985a) and the study of gyrotrons (e.g., Sprangle and Drobot, 1977;Read, Chu, and Dudas, 1982). In this process, electrons are phase bunched via gyroresonant interactions with a leading, coherent wave packet.…”

“…In this process, electrons are phase bunched via gyroresonant interactions with a leading, coherent wave packet. After this leading wave packet has passed the electrons, the current associated with the phase-bunched electrons radiates coherently (Sprangle and Drobot, 1977;Winglee, 1985a;Melrose, 1986). The frequency at which the electrons radiate is determined by the magnetic field with which they interact.…”