Gyrokinetic Particle Simulation of Nonlinear Saturation of Mirror Instability

Low frequency compressible electromagnetic mirror modes driven by temperature anisotropy in high-beta plasmas have been observed by satellites in space plasmas, such as planetary and cometary magnetosheaths. In this work, mechanism of the nonlinear saturation of the mirror instability is studied using the gyrokinetic particle simulation. Phase-space particle trapping due to the nonlinear mirror force is found to be the dominant saturation mechanism in the simulation of a single mirror mode with relatively weak drive [Nonlinear Saturation of Mirror Instability, H. Qu, Z. Lin, and L. Chen, Geophy. Res. Lett. 35, L10108 (2008)]. At the nonlinear saturation, the phase-space island of the distribution function is formed. The oscillation frequency of the saturated perturbation amplitude is close to the bounce frequency of the trapped particles, which is comparable to the linear growth rate of the mirror mode. Scaling of the saturation amplitude is consistent with the onset of the particle trapping. With strong instability drive, relaxation toward marginal stability dominates the nonlinear saturation of the mirror instability. Phase-space trapping, however, persists after the saturation and continues to regulate the nonlinear evolution of the mirror mode. Applications of the gyrokinetic particle simulation for the studies of nonlinear kinetic processes in space plasmas, such as solar wind heating by Alfvenic turbulence and excitation of low frequency drift compressible modes, will also be discussed. This work is supported by an NSF CAREER Award.