We studied the coherence and phase structure of Pc 3 pulsations in the H and D components of the magnetic field observed at latitudinally distributed stations along the 210¿ magnetic meridian by using the H component at GUA (L=1.01) as the reference. At GUA, which is located near the magnetic equator, field line resonance does not occur because of the small vertical component of the ambient magnetic field, so pulsations observed there are likely produced by fast mode waves propagating from sources external to the magnetosphere. We found that the D-component Pc 3's in the prenoon sector 0900--1200 LT at L=1.2--5.5 show a coherent, in-phase (out-of-phase) oscillation in the northern (southern) hemisphere with respect to the H component Pc 3's at GUA. This result indicates that the D-component Pc 3's in the prenoon sector at L=1.2--5.5 are closely related to the upstream source waves. ¿ 1998 American Geophysical Union