Freja UV imager observations of spatially periodic auroral distortions

Freja UV imager measurements of distortions of discrete arcs within the auroral distribution reveal a number of examples of periodic intensifications. Analysis of these features reveals a clear peak at 152 km separation between the intensifications with a less well defined peak at approximately twice that value. No observations at separations less than 100 km were found. Theories relating such features to shear flow regions suggest a relationship between wavelength and shear layer width. A statistical analysis indicates that the product of the wave number and the arc system width yielded a value of .7 which is consistent with theoretical predictions. The periodic features were found to occur throughout the afternoon/midnight sector with no apparent dependence on MLT. As well they appeared at a variety of locations within the auroral distribution implying the common existence of shear layers.