Formation of electron beams by the interaction of a whistler wave packet with radiation belt electrons

The purpose of this paper is to discuss the properties of electron beams fo rmed by cyclotron interactions between radiation belt electrons and a quasi -monochromatic whistler wave packet from a ground-based VLF transmitter. Th e beams are formed due to trapping of the electrons at the forward edge of the wave packet, their acceleration inside the wave packet, the escape of t he accelerated electrons from the moving backward edge of the wave packet, and their following free motion in an inhomogeneous magnetic field. A combi nation of these processes provides the main features of the spatial-tempora l evolution of the beams which are investigated both analytically and numer ically. It is shown that one or two beams can appear at one point at the sa me time, and that the density of the beams increases during their expansion . Motion of the pumping wave packet in the inhomogeneous magnetic field pro vides the variations of the initial velocity and position of the beam injec tion which change the spatial and temporal gradients of the parallel veloci ty of the beam, in contrast with the case of the pure adiabatic motion of a n individual electron. Such a behaviour can be significant for the generati on of secondary emissions. Numerical calculations demonstrate a wide variet y of the spatio-temporal patterns of the beam parallel velocity depending o n the plasma and wave packet parameters. It is shown that the most signific ant parameters which determine the beam characteristics are the wave packet length about the equator, its group velocity, and the initial energy and p itch angle of the electrons. (C) 2000 Elsevier Science Ltd. All rights rese rved.