Hybrid Simulations of Nonlinear Magnetospheric Alfven Waves

A two-dimensional magnetohydrodynamic-gyrokinetic hybrid particle-in-cell initial-value code has been developed to simulate the nonlinear evolution of Alfven-ballooning modes destabilized by the drift-bounce resonance of energetic particles in the Earth's magnetosphere. The physical model is based on the analytical formulation developed for two-component (core and energetic) plasmas by Chen and Hasegawa [1991]. In the conditions of interest, the core component supports the magnetohydrodynamic Alfven oscillations, while the energetic component provides the instability drive. Recent one-dimensional linear investigations have numerically demonstrated the energetic particle resonant excitation along a single field line [Dettrick, Chen, Zheng, 2002]. In such linear studies, the energetic particles are taken to follow the unperturbed equilibrium phase-space trajectories. As the unstable modes evolve into the nonlinear state, the finite-amplitude perturbations may significantly alter the phase-space trajectories of the resonant particles. In particular, there will be perturbed radial drifts of particles, resulting in the spatial detuning of the wave-particle resonant condition and, hence, possible nonlinear saturation. The relevant features and theoretical results will be discussed in details. Work supported by U.S. DOE and NSF Grants.