Fine structure of optical aurora

This is a review paper on the fine structure of optical aurora. Spatial scales smaller than about 1 km and temporal scales shorter than about 1 s are considered. Fine structure is present in most types of aurora, but earlier much of it has not been possible to study properly due to instrument limitations. Recent advancements in optical instrumentation have provided considerable improvements of temporal and spatial resolution. Optical measurements are able to give higher resolution than any other types of ground-based instruments used in auroral studies. To interpret the information, both more advanced modelling and analysis methods are being developed. This gives access to new knowledge on the physical processes responsible for particle acceleration, precipitation, atmospheric ion chemistry, and auroral light production.