Observations of a monochromatic Pc 5 ULF pulsation event with the simultaneous occurrence of resonance at two latitudes are presented. The event indicates the presence of a large inward directed postive gradient in the equatorial cold plasma density. Model calculations are used to deduce the radial variation of the plasma density. The results support the idea that resonant Pc 5 pulsations are driven by an inward propagating wave, which may be generated at the magnetopause by the Kelvin-Helmholtz instability.