The low energy particle signature of substorm dipolarization is exhibited through a case study of RIMS data on DE-1 at ~2100 MLT, ILAT=59¿--65¿, L=3.8--5.4 RE, and geocentric distances 2.6--2.9 RE. A strong cross-field-line, poleward outflow that lasts for a few minutes with a velocity that reaches at least 50 km/s is correlated with substorm activity evidenced in the AE index and the MAG-1 data. All the major species (H+, He+, O+) are observed to have the same bulk velocity. The parallel velocities are strongly correlated with the perpendicular velocities. The parallel acceleration is shown to result from the centrifugal force of the E¿B drift induced by the dipolarizing perturbation of the magnetic field.