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Experiment Observations and New Theoretical Approaches to Nonlinear Wave Growth in Magnetospheric Chorus and Triggered Emissions

The nonlinear whistler mode instability associated with magnetospheric chorus and VLF triggered emissions is known to be a key driver in magnetospheric dynamics. Nevertheless, despite many years of dedicated experiments, observations, theories, and simulations, the process continues to be poorly understood. We present a review of key observations and following up on recent formulations of other authors, we give a new analytical exploration of the stability of the phenomenon. This exploration derives an additional requirement on the anisotropy of the energetic electron distribution relative to the linear treatment of the instability, and shows that the nonlinear instability is strongest when electrons become initially trapped in the wave potential. These results imply that the initiation of the nonlinear instability at the equator requires a positive frequency sweep rate, while the initiation of the instability by a constant frequency triggering wave must occur at a location upstream of the geomagnetic equator.