  Electromagnetic ion cyclotron waves are known to exhibit frequency chirping,
contributing to the rapid scattering and acceleration of energetic particles.
However, the physical mechanism of chirping remains elusive. Here, we propose a
new model to explain the chirping and provide direct observational evidence for
validation. Our results relate the frequency chirping of the wave to both the
wave amplitude and magnetic field inhomogeneity for the first time. The general
applicability of the model's underlying principle opens a new path toward
understanding the frequency chirping of other waves.
