Optical observations using high-resolution television cameras frequently show that auroral curls are associated with shear velocities in the apparent optical flow. The present study examines in detail one particular curl system event which happened to yield sufficient resolution to determine the fine structure of velocity and vorticity profiles by means of a new analysis technique. Those observations of curl system evolution are contrasted with large velocity shear events where small-scale quasiperiodic distortions were subject to sudden decay rather than development into vortices. The results are discussed in light of an electrostatic picture of auroral acceleration and the Kelvin-Helmholtz instability model. We suggest that the latter cannot fully explain the nonlinear phase of the observed curl system event and that curl models should take auroral acceleration processes into account. ¿ 1999 American Geophysical Union