Nonlinear dynamics of resonant interactions between wave packets and particle distributions with loss-cone-like structures

Abstract

The paper studies the nonlinear mechanisms at work in magnetized plasmas when wave packets interact resonantly with particle distributions presenting loss-cone-like structures. Lower hybrid waves are considered in view of the great importance, in space and laboratory plasmas, of waves with frequencies below the electron cyclotron frequency. Owing to a three-dimensional Hamiltonian model and a numerical symplectic code, the authors study the nonlinear stage of the loss-cone instability for various particle distributions and wave spectra involving symmetric and asymmetric features. In particular, the wave-particle interaction process of dynamical resonance merging, which results from an instability of the trapped particles' motion and leads to complex stochastic phenomena, is discussed. Whereas interactions at normal cyclotron resonances are mostly considered, the role of the Landau and the anomalous cyclotron resonances is also studied to explain thoroughly the nonlinear wave-particle dynamics as well as the competition between loss-cone, fan, and beam instabilities. The relaxed particle distributions and the saturated wave spectra are analyzed. The time necessary for filling the loss-cone structures is determined as a function of the characteristics of the particle distributions. Whereas most of the previous works analyzed the asymptotic stage of the system's evolution in the frame of the well-known quasilinear theory, the paper considers the case when the energy carried by the wave packet is sufficiently large so that the description of the physical processes at work cannot be limited to the frame of weak turbulence theories.