Saturation effects in the VLF‐triggered emission process

Abstract

[1] An in-depth study is performed of the saturation characteristics of the VLF-triggered emission process, a plasma instability associated with the amplification of whistler mode signals in the magnetosphere. A survey of data from the 1986 operating year of the Siple VLF wave injection experiment reveals that long-period oscillations (characterized by a pattern of growth to saturation then subsequent suppression of the wave), shortperiod oscillations (previously identified as sidebands), and generation of incoherent wave energy around the frequency of the triggering signal are all characteristics of the instability at saturation. A model is developed to study these phenomena. The model is in some respects similar to the Vlasov Hybrid Simulation but modified so that saturation is a natural consequence of the modeled growth process. Results from the model indicate that the growth and eventual saturation are caused by wave amplitude gradients, most notably gradients that indicate a transition from an amplitude where the wave cannot trap electrons in its potential in the presence of the inhomogeneous magnetospheric magnetic field to an amplitude where such trapping is possible. That is, resonant currents are enhanced in the region where the phase space electron hole created by the trapping condition is allowed to mix with the ambient energetic electrons. This growth process is found to naturally lead to saturation of the instability.

Triggering process of whistler mode chorus emissions in the magnetosphere

“…The VHS method suffers from neither of these problems. The code by Gibby et al [2008] does not update wave phase properly when wave amplitude is very small and for a broadband simulation has a rather low resolution in phase space, which may be why the code does not trigger emissions. Gibby et al [2008] presents interesting and plausible data, supported by his simulations, suggesting that in the key down case saturation may arise from marked spectral broadening which destroys particle trapping and thus nonlinear growth rates.…”

Preferential amplification of rising versus falling frequency whistler mode signals

“…The hot plasma distribution function is a bi‐Maxwellian with a temperature anisotropy of 1.16 and a hot electron density ( n h ) of 0.25 cm −3 . The simulation domain extends out ±5000 km around the equator, chosen based on the minimum trapping amplitude threshold parameterized by S and for consistency with past work [ Nunn , ; Gibby et al , ]. Pulses that rise or fall in frequency are injected into the simulation space by enforcing a chirped phase at the input with an initial amplitude of 0.1 pT.…”

CLUSTER and IMAGE: New Ways to Study the Earth’s Plasmasphere

“…The introduction of the concept of a wave distribution function (Storey andLefeuvre 1979, 1980) heralded the beginning of efforts to track waves observed on spacecraft to their regions of origin. A lot of work was devoted to the study of non-linear whistler-mode wave-particle interactions using a ground-based transmitter in Antarctica (Helliwell and Katsufrakis 1974;Paschal and Helliwell 1984;Helliwell 1988), supported by theoretical modeling (e.g., Nunn 1974;Omura and Matsumoto 1982;Gibby et al 2008). The loss of radiation belt particles through scattering by whistler-mode waves has been studied by, e.g., Inan et al (1978), Burgess and Inan (1993), and Abel and Thorne (1994).…”