Fine structure of aurora

Fine structure is present in most types of aurora, but much of it has previously not been possible to study properly because of instrument limitations. However, recent advances in optical instrumentation have provided considerable improvements in temporal and spatial resolution. Optical measurement systems are able to use a higher resolution than other types of ground-based instruments used in auroral studies. New results have been obtained regarding, for example, elemental structures in discrete auroras, generation of flickering aurora, generation of Alfvén waves in shear regions, dynamic rayed aurora, fine structure of diffuse auroras and fine structure of auroral curls. Outstanding questions are highlighted and recommendations for future research are given. The importance of a coordinated infrastructure for ionospheric research, simultaneous measurements on different scales, optical calibration facilities and the development of time-dependent high-resolution models is stressed.