Three-dimensional (3-D) particle simulations are used to investigate the response of the near-Earth plasma sheet to the imposition of an external convection electric field for the case where magnetic flux is blocked from leaving the region. Under these conditions an interchange mode with characteristic wavelength of 1--2 RE (determined by the ion gyroradius) in the east-west direction is excited before the plasma sheet is severed by the formation of a neutral line. The nonlinear growth of the interchange mode produces a strong (10--20 mV/m), localized (≲0.5 RE) Ey field which drives earthward flows of both electrons and ions. These flows pinch off regions of southward Bz which eventually are expelled tailward in narrow fingers around the edges of the earthward flow bursts. ¿ 1997 American Geophysical Union