Amplitude Variations in Artificially Stimulated v.l.f. Emissions

Abstract

AN understanding of the basic mechanisms involved in wave-particle interactions in a magneto-active plasma is currently of interest in contexts ranging from planetary and pulsar magnetospheres to galactic jets. In the Earth's magnetosphere, which is an excellent laboratory for the study of an infinite collisionless plasma with an inhomogeneous magnetic field, ground and satellite observations of discrete very low frequency (v.l.f.) emissions are providing information on the interaction of waves in the whistler mode with asymmetric velocity distributions of energetic electrons in the radiation belts. The emissions observed consist of rising, falling, hook-shaped, or approximately steady tones, triggered in the equatorial region of the magnetosphere by natural whistlers, by unknown natural sources, by short (150 ms) pulses from high power (106 W) ground transmitters1, or by long (∼ 1 s) pulses from low power (∼ 100 W) ground transmitters of the Omega network2. The basic properties3 of the emissions to be accounted for on a microscopic theory are the narrow bandwidth, the frequency variations, the amplitudes observed (sometimes an order of magnitude greater than the amplitude of the triggering pulse), the triggering delay of order 100 ms and the fact that for ground observations triggering is generally seen at frequencies ω such that ω≈0.5 Ω0, where Ω0 is the equatorial value of the electron gyrofrequency on the appropriate field line. I shall report here the observation of an additional property: large regular and irregular classes of amplitude fluctuation within artificially stimulated v.l.f. emissions triggered by low power Omega pulses of frequency 10.2 kHz received on the ground in the magnetically conjugate zone of the transmitter after following a magnetospheric trajectory.