The Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE satellite frequently observed ULF pulsations in the Pc 3--4 period range as it traveled from L=3 to beyond L=8 near local noon. In addition to the azimuthally polarized, harmonically structured pulsations that occurred frequently at all dayside magnetic latitudes sampled by the satellite (¿16¿), a distinctive class of pulsations appeared occasionally during quiet times when the satellite was within 1¿ or 2¿ of the geomagnetic equator. These pulsations, with increased amplitude (up to 12 nT peak to peak) and without multiple harmonic structure, were also transverse but with dominant power in the radial component. They were observed during magnetically quiet periods in regions characterized by intense fluxes of warm, strongly trapped light ions, predominantly H+, and often with streaming low-energy plasma. We interpret the strong latitudinal localization of these pulsations to be due to equatorial mass loading or to partial reflection of Alfv¿n wave energy by latitudinal gradients in plasma density. In most cases we have identified a recently injected energetic particle population at the satellite's location as the probable source of wave energy. The similarity in occurrence between these events and the ''giant'' pulsations occasionally observed on the ground suggests that latitudinally localized plasma structures may be a necessary but not sufficient condition for the onset of these unusually long-lived, highly localized pulsations. ¿ American Geophysical Union 1988