Using a spatially two-dimensional electrostatic particle simulation code, we examine the stability of a plasma equilibrium characterized by a localized transverse dc electric field and a magnetic-field-aligned electron drift for L≪Lx, where Lx is the simulation length in the x direction and L is the scale length associated with the dc electric field. It is found that the dc electric field and the field-aligned current can together play a synergistic role to enable the excitation of electrostatic waves even when the threshold values of the field-aligned drift and the E¿B drift are individually subcritical. The simulation results indicate that a broadband turbulence is associated with such an equilibrium. This is in contrast to the current-driven ion cyclotron instability, which is characterized by a coherent spectrum around the ion cyclotron harmonic. Further, the growing ion waves are associated with small vortices in the linear stage, which evolves to a nonlinear state dominated by larger vortices with lower frequencies. ¿ American Geophysical Union 1990