Geosynchronous orbit data from charged particle analyzer (30- to 300-keV electron flux) and magnetometer instruments on board the 1984-129, GOES 6, GOES 5, and 1982-019 satellites distributed in the midnight-to-dawn sector are examined during a particle injection event that occurred at 0820 UT on May 21, 1986. The injection was detected by 1984-129 in the midnight sector as a dispersionless and short-lived (duration was 9 min) event. The injected electron cloud drifted eastward and was detected by 1982-019 in the dawn sector. Meanwhile, the GOES 6 and 5 satellites were in the postmidnight sector between the satellites which measured the electron flux. They observed transient field line oscillations during the periods when the azimuthal plasma pressure arising from the electron cloud was in the postmidnight sector. It is suggested that field-aligned currents are likely to be built up preferentially by a divergence of diamagnetic currents in the region where the azimuthal plasma pressure gradient dominates. It is argued that these currents excite an azimuthally polarized transient field line oscillation. ¿ American Geophysical Union 1996