First Observation of Four‐Band Ion Cyclotron Whistlers

Abstract Ion cyclotron whistlers are frequently observed in low Earth orbit, typically formed when right‐hand polarized lightning whistlers undergo polarization reversal at crossover frequencies. In this study, we for the first time report a four‐band ion cyclotron whistler event detected by Van Allen Probe A. We analyze their fine structures and propagation characteristics, presenting the first observational evidence of ion cyclotron whistlers undergoing multiple ionospheric reflections. Using spectral features of electron whistlers and four‐band ion cyclotron whistlers, we infer the vertical extent of four ion species. The results suggest that they exist at sufficient densities from specific altitudes up to the satellite to resonate with and absorb ion cyclotron waves: H + from 400 to 500 km, M/Q = 2 from ∼900 km, He + from ∼700 km, and M/Q = 8 from ∼800 km, respectively. This multi‐band analysis of ion cyclotron whistlers provides a new perspective on using them as diagnostic tools for inferring ion presence.