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 f_o to their natural modes at f_o + 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.

When two coherent signals are transmitted simultaneously with a small frequency spacing, they may interact with one another. The critical separation, called the coherence bandwidth (CB), is typically near 50 Hz and leads to estimated values for the wave intensity of 2.5–10 pT, comparable with those estimated from the BLI.

When the transmitter output power is reduced below some critical level (varies with time), temporal growth ceases. This initial power level depends on the individual ducted path and may be related to the signal attenuation from transmitter to IR on the path.