Collisional modes driven by sher in the plasma flow velocity V, parallel to the magnetic field, are shown to exist over significant scale distances while subject to the constraints imposed by gradients in the transverse velocity VE∝E¿B. These constraints make the modes localized over finite distances around the surfaces where VE reaches a maximum or a minimum. Composite modes that can be constructed as a sequence or a superposition of these ''elementary'' normal modes can be excited in regions where the transverse velocity acquires a plateau type of profile that is assumed to be formed after macroscopic Kelvin-Helmholtz modes associated with the shear of VE have reached their saturation state. Thus a significant rate of longitudinal plasma momentum transport in the transverse direction to the magnetic field can be produced. The relevance of this analysis of the fluctuations observed in the auroral F region is discussed. ¿ American Geophysical Union 1990