Nonlinear Wave Growth Theory of Coherent Hiss Emissions in the Plasmasphere

Abstract:

Recent observations of plasmaspheric hiss emissions by the Van Allen Probes show
that broadband hiss emissions in the plasmasphere comprise short-time coherent elements
with rising and falling tone frequencies. Based on nonlinear wave growth theory of
whistler-mode chorus emissions, we have examined the applicability of the nonlinear
theory to the coherent hiss emissions. We have generalized the derivation of the optimum
wave amplitude for triggering rising tone chorus emissions for
both rising and falling tone hiss elements.
The amplitude profiles of the hiss emissions
are well approximated by the optimum wave amplitudes for triggering rising or falling
tones. Through the formation of electron holes for rising tones and electron hills for
falling tones, the coherent waves evolve to attain the optimum amplitudes.
An electromagnetic particle simulation confirms
the nonlinear wave growth mechanism as the initial phase of the hiss generation process.
We find very good agreement between the theoretical optimum amplitudes and
the observed amplitudes as a function of instantaneous frequency.
We calculate nonlinear growth rates at the equator, and find that nonlinear growth
rates for rising-tone emissions are much larger than the linear growth rates.
The time scales of observed hiss emissions also agree with
those predicted by the nonlinear theory.
Based on the theory, we can infer properties of
energetic electrons generating hiss emissions in the equatorial region of
the plasmasphere.