Statistical Investigation of the Interplay Between Chorus and Electron Cyclotron Harmonic Waves

Whistler-mode chorus and electrostatic electron cyclotron harmonic (ECH) waves are fundamental plasma modes in Earth's magnetosphere that play crucial roles in radiation belt dynamics. While their individual characteristics have been extensively investigated, systematic statistical analyses of their interplay and its dependence on ambient plasma conditions remain scarce. Using observations from the Van Allen Probes between October 2012 and December 2014, we identified 8,750 ECH-only, 4,215 chorus-only, and 12,467 co-occurring events. Simultaneous chorus and ECH waves are frequently observed near the magnetic equator, primarily in the midnight-to-dawn sector at L-shells between 4 and 6. These co-occurring events are more prevalent and display stronger wave amplitudes than intervals in which only one wave type is present. During co-occurring intervals, their amplitudes exhibit a clear anticorrelation, indicating that chorus-ECH interactions are ubiquitous in the Earth's magnetosphere. Moreover, the amplitude ratio between chorus and ECH waves decreases with increasing plasma-to-cyclotron frequency ratio ((Formula presented.)) and hot electron density but shows little dependence on electron temperature anisotropy or parallel plasma beta. These findings provide new statistical evidence for the widespread occurrence of chorus-ECH interactions and highlight their significance for electron dynamics and space weather modeling.