We present Active Magnetospheric Particle Tracer Explorers Ion Release Module (AMPTE IRM) observations of a wave packet of compressional Pc5 ULF oscillations between 0915 and 1030 UT on October 28, 1984. The waves are observed in the local morning near the equatorial plane, have a dominant period of ~380 s, and most probably have a fundamental field-aligned harmonic structure. The waves have previously been interpreted as a magnetospheric waveguide mode propagating downtail [Mann et al., 1998>. At the time of the maximum amplitude of the waves, AMPTE IRM observes oscillations in the background density at a frequency twice that of the coincident ULF pulsations. We develop a theory which explains the generation of frequency-doubled density fluctuations through large-amplitude radial displacements of curved flux tubes in a nonadiabatic background plasma pressure distribution. The theory predicts a phase locking between the peaks and troughs of the radial velocity field and the peaks in the frequency-doubled density oscillations which is observed by AMPTE IRM. The observations strongly support the hypothesis that the frequency doubling occurs as a result of finite amplitude effects. ¿ 1999 American Geophysical Union