X ray images of the Earth's auroral zone give a large-scale view of the precipitation patterns of energetic electrons through the bremsstrahlung produced by the electrons stopping in the atmosphere. Here we present 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 features 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 along the track of the satellite. The technique was used to study 26 auroral 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 conclusion that significant spectral variations occur on kilometer size scales. ¿ American Geophysical Union 1993