Ring Current Ion Scattering by Electromagnetic Ion Cyclotron Waves: Nonlinear Resonance Effects

Electromagnetic ion cyclotron (EMIC) waves can interact with ring current ions and scatter them into the atmosphere. The classical framework for studying EMIC wave–ion interaction is the quasi‐linear theory, which relies on the approximation of weak perturbations of particle trajectories for a single resonant interaction. However, intense EMIC waves are frequently observed in the magnetosphere, and such waves can significantly alter particle trajectories, thereby violating the assumptions of quasi‐linear theory. In this study, we first use test‐particle simulations to demonstrate the characteristics of nonlinear effects for different ion species interacting with EMIC waves. We then apply a mapping technique to investigate the long‐term wave‐particle interactions. The mapping technique is an analytical approach based on the theoretical properties of both linear and nonlinear interactions. It can efficiently trace the long‐term dynamics of a large population of ions across both interaction regimes without requiring extensive computational resources. Using this technique, we analyze the long‐term behavior of ions interacting with H‐band EMIC waves and compute the corresponding nonlinear diffusion coefficient. We further derive a rescaling factor for quasi‐linear diffusion rates. This rescaling factor enables quasi‐linear models to be extended to incorporate nonlinear effects.