This paper demonstrates how ion cyclotron waves generated in the equatorial magnetosphere can propagate to the ground and resolves a discrepancy between theory and observation that has lasted over a decade. We present a full wave analysis for ion cyclotron waves propagating from the equatorial magnetosphere to the ground. Our analysis demonstrates that strong mode coupling occurs near the He+ and O+ resonance locations which allows the equatorial waves to penetrate to ionospheric altitudes with substantial amplitude and shifted polarization consistent with observational studies. ¿ 1999 American Geophysical Union