A new variety of fine frequency structure of some Pc1 geomagnetic pulsations has been recently reported. For these pulsations, each periodic element consists of a chevron-shaped combination of rising and falling tones. A careful spectral analysis of one of these events, based upon a new numerical technique known as the Modified Moving Window Method (MMWM) allows us to establish the reality of this structure. This is tentatively interpreted in terms of propagation in a magnetospheric plasma containing a non negligible fraction of helium ions, the presence of helium ions resulting in a minimum in the ULF group velocity between the ion hybrid and ion crossover frequencies.