We present evidence of filamented structure in the auroral ionosphere, observed through enhanced radar echoes produced by plasma instabilities in the filaments. Enhancements are observed in up- or down-shifted ion-acoustic peaks, or both, with power well above thermal levels. Detailed theoretical understanding of these enhancements is still lacking, and several different theories are currently used to explain the observations. Using an interferometric technique, we have measured the horizontal scales of the structures, and their evolution in time, with unprecedented resolution. To explain simultaneous up- and down-shifted enhancements using theories that could only enhance one of these shoulders, spatial and/or temporal averaging has previously been suggested. However, the present observations show that enhancements occur simultaneously and in the same volume. The observed scale size in the plane perpendicular to the magnetic field is comparable to the smaller scale size of optical aurora, which, despite extensive attempts, has not been successfully explained.