Shear velocity profiles associated with auroral curls

Optical observations using high-resolution television cameras frequently sh ow that auroral curls are associated with shear velocities in the apparent optical flow. The present study examines in detail one particular curl syst em event which happened to yield sufficient resolution to determine the fin e structure of velocity and vorticity profiles by means of a new analysis t echnique. Those observations of curl system evolution are contrasted with l arge 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 can not fully explain the nonlinear phase of the observed curl system event and that curl models should take auroral acceleration processes into account.