KILOMETER SCALE STRUCTURES IN AURORAL X-RAY ARCS DETECTED BY COMBINING SATELLITE X-RAY IMAGES WITH INSITU ENERGETIC ELECTRON FLUX PROFILES

X ray images of the Earth's auroral zone give a large-scale view of th e precipitation patterns of energetic electrons through the bremsstrah lung produced by the electrons stopping in the atmosphere. Here we pre sent X ray maps based on observations by the XRIS imager on the S81-1 polar orbiting satellite; discrete X ray arcs are the most prominent f eatures in these data. The full width half maximum spatial resolution of XRIS was 40 km, but high time resolution measurements of electrons above 16 keV could detect 2 km spatial variations in one dimension alo ng the track of the satellite. The technique was used to study 26 auro ral arcs observed in 4-40 keV X rays by the XRIS and showed that in 2/ 3 of the cases the full width of the precipitation region was smaller than 40 km. Furthermore, comparisons with electron data lead to the co nclusion that significant spectral variations occur on kilometer size scales.