Using a ten‐chord interferometer, we have measured a field‐aligned (k∥≊2 m−1;k⊥≳25 m−1) plasma‐density disturbance propagating alongBwith a speed in the ion acoustic range. The propagation is purely in the electron drift direction and is observed only when the drift parameter (electron drift speed/electron thermal speed)≳0.1. A novel spatial‐filter technique resolves this localized mode, which otherwise would be hidden by more robust global disturbances present along the lines of sight.