Global Modulation of Ring Current Protons and EMIC Waves by Ultralow-Frequency (ULF) Wave–Driven Radial Diffusion

Ultra-low-frequency (ULF) waves contribute to particle acceleration and scattering via radial diffusion and modulate the periodicity of electromagnetic ion cyclotron (EMIC) waves. Previous studies primarily focused on individual wave modes, leaving the global-scale effects of ULF waves on EMIC wave instability poorly understood. Using the ring current model STRIM, we investigate the global effects of ULF wave-driven radial diffusion on EMIC wave growth rates during storms. Results show that ULF-driven radial diffusion enhances EMIC growth rates in the vicinity of proton flux peaks while suppressing them at the peak centers, broadening the EMIC excitation region. Comparisons with in situ observations indicate that including the effects of ULF waves improves the model's ability to reproduce the intensity and distribution of EMIC growth rates. These findings highlight the critical role of ULF waves in shaping EMIC excitation regions and modulating proton dynamics.