Observations of distortions of optical features in the UV auroral distribution

Observations made by the Viking satellite have for the first time shown the development of multiple large-scale surge features. The longer time scale of the features is contrasted with smaller-scale vortex streets found along the poleward edge of the evening oval, which are generally observed to exist for less than 2 min. The ratio of wavelength-to-diameter of these features is shown to decrease with increasing wavelength. These observations support the view that the origin of the larger surge features are of the Kelvin-Helmholtz instability type, originating further out in the magnetosphere than their vortex-street counterparts. Observations of spiral features have been made at all local times and the spirals always have a counterclockwise sense (viewed in the direction of the magnetic field in the northern hemisphere). One interpretation of these events is that they are the result of an exponential growth along a field line of an initial upward field-aligned current perturbation