Frequency Chirping of Electromagnetic Ion Cyclotron Waves in Earth's Magnetosphere

Electromagnetic ion cyclotron (EMIC) waves are known to exhibit frequency chirping occasionally, contributing to the rapid acceleration and precipitation of energetic particles in the magnetosphere. However, the chirping mechanism of EMIC waves remains elusive. In this work, a phenomenological model of whistler mode chorus waves named the Trap-Release-Amplify (TaRA) model is applied to EMIC waves. Based on the proposed model, we explain how the chirping of EMIC waves occurs, and give predictions on their frequency chirping rates. For the first time, we relate the frequency chirping rate of EMIC waves to both the wave amplitude and the background magnetic field inhomogeneity. Direct observational evidence is provided to validate the model using previously published events of chirping EMIC waves. Our results not only provide a new model for EMIC wave frequency chirping, but more importantly, they indicate the potential wide applicability of the underlying principles of TaRA model. Rapid change of wave frequency, or frequency chirping, can frequently be observed in space and laboratory plasmas. Chirping waves generally appear as discrete and narrowband elements and can cause rapid acceleration or scattering of energetic particles through nonlinear interactions. Correspondingly, the physical mechanism of frequency chirping has attracted considerable research interest. This work aims to explain how electromagnetic ion cyclotron (EMIC) wave frequency chirping arises, which remains an open question. We apply a previously developed model of whistler mode chorus waves named the Trap-Release-Amplify (TaRA) model to chirping EMIC waves, and make a series of chirping rate predictions. In particular, we find that after taking into account all the different stages of wave excitation, chirping rate of EMIC waves should be related to both wave amplitude and background magnetic field inhomogeneity. A comparison between the theoretical chirping rates and observations using previously published EMIC wave chirping events shows good consistency. The results provide both a new explanation for EMIC wave frequency chirping and an indirect test of the underlying principles of the TaRA model. We propose and validate a model of electromagnetic ion cyclotron (EMIC) wave frequency chirping based on the Trap-Release-Amplify model of chorus waves The model relates EMIC wave frequency chirping to both wave intensity and background magnetic field inhomogeneity for the first time Observations demonstrate a clear decrease in EMIC wave chirping rate with increasing L-shell, consistent with the model prediction