A series of three structured Pc 1 events were observed prior to the July 29, 1977, sudden commencement by magnetometer stations at College, Alaska, MacQuarie Island, Australia, and Vostok, Antarctica. Signal characteristics of the events lead us to infer that the near-conjugate College and MacQuarie Island stations were near the source field lines for the second of these events. From the growth rates for ion cyclotron waves calculated from linearized equations for an anisotropic, drifting plasma we have inferred the presence of freshly injected plasma in the afternoon magnetosphere after 2330 UT at L=5.4. This is consistent with the increased convection to be expected with the observed southward turning of the interplanetary magnetic field prior to the sudden commencement.