It is shown that the electrojet velocity is reduced to the ion acosutic velocity by the two-stream irregularities if the width of the unstable layer is larger than say, several hundred meters. However, if it is of the order of 100 m or smaller, the electrojet velocity is not appreciably changed, but the electron density is modified, the instability thereby being stabilized. On the basis of these results it is postulated that electrojets responsible for diffuse radar auroras are occasionally highly irregular and consist of many narrow streams with widths of the order of 100 m. Then the observational fact that irregularities with velocities larger than the ion acoustic velocity exist can be explained.