Saturation effects in the VLF-triggered emission process

摘要

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), short-period 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.