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ONE of the important problems in the field of pc 1 micropulsation (pearl) research is to determine what portion of the magnetosphere is involved in the propagation of an individual structured pc 1 event. It is generally recognized1that these micropulsations propagate back and forth in a dispersive medium2,3between conjugate hemispheres guided by the Earth's magnetic field, but it is not known whether the waves are constrained to propagate along a single field line or whether they can spread out along an appreciable portion of a magnetic shell. Because energy is transferred back and forth between trapped particles and ultra-low frequency (ULF) waves in the cyclotron resonance interaction4, and because trapped particles undergo longitudinal drift, it is reasonable to assume that a structured pc 1 micropulsation may spread out along a magnetic shell even if it started on a particular field line. Dispersion measurements5–8which have been used to locate the latitude where the field lines or shells intercept the surface of the Earth cannot be used to determine longitude, and hence one must resort to interstation comparison of data.