Fine-scale optical observations of aurora

High spatial and temporal resolution observations of auroral forms made by the University of Calgary Portable Auroral Imager are reviewed. Observation s include auroral forms with apparent widths of on the order of tens to hun dreds of meters and lifetimes of less than a second to several tens of seco nds. Included here is a class of multiple auroral are systems exhibiting a strikingly asymmetric internal topology. It is shown that such systems of m ultiple linear arcs may be explained by mode conversion of field-line reson ance shear Alfven waves to inertial Alfven waves. A survey of small-scale, short-lived auroral vortices (curls) occurring on thin auroral forms within the active aurora has also been performed, with some results being reviewe d here. The Kelvin-Helmholtz instability as a responsible mechanism is disc ussed in light of the observationally acquired statistical data. Observatio ns of small-scale "black" auroral arcs and vortices are briefly reviewed as well. (C) 2001 Elsevier Science Ltd. All rights reserved.