Siple Station Experiments on Wave-Particle Interactions in the Magnetosphere

Abstract

Natural and controlled whistler-mode signals have been used at Siple Station (and its conjugate, Roberval, Quebec) to study nonlinear mechanisms of wave growth and wave-wave interactions (WWI) in the magnetosphere. Three general classes of WWI (triggering, suppression, and entrainment) are identified and interpreted in terms of a model based on cyclotron resonance interaction. A new type of triggered emission, the ‘band-limited impulse’ (BLI), often appears at the end of an amplified signal. It covers a frequency range of about 150 Hz above the carrier. It is interpreted in terms of the switching of phase-bunched currents from their driven mode at the carrier frequency fo to their natural modes at fo + Δf where Δf depends on the change in medium parameters over the interaction region. In a related type of BLI, which appears before the termination of the amplified pulse, the frequencies are symmetrically distributed about the carrier. Assuming that this BLI is caused by wave-induced spread in the vM of the interacting electrons, the frequency range (± 100 Hz) of the BLI gives an estimate of the local values (~20 pT) of the wave field. Triggering and entrainment of self- sustained emissions are interpreted using the same model and their properties lead to independent estimates of signal strength (~1 pT) at the input to the interaction region (IR). Measured temporal growth typically ranges from 20–30 dB, giving output fields of 10–100 pT in the equatorial region at L = 4. It is estimated that a step function wave of this value can produce an initial pitch precipitation flux of ~0.1 ergs/cm2-sec for E≥1 keV through pitch angle scattering, sufficient to explain observed transient bursts of X-ray fluxes and E-region ionization enhancements.