Electron density profiles acquired with the EISCAT radar at 0.2 s time resolution, together with TV images and photometric intensities, were used to study the characteristics of thin (<1 km) auroral arc structures that drifted through the field of view of the instruments. It is demonstrated that both high time and space resolution are essential for deriving the input parameters of the electron flux responsible for the elemental auroral structures. One such structure required a 400 mW m-2 (erg cm-2 s-1) downward energy flux carried by an 8 keV monochromatic electron flux equivalent to a current density of 50 &mgr;A m-2. ¿ American Geophysical Union 1994