GRaNDScan An Experiment to Study Cosmic Ray Flux and Anisotropy around and below EeV

For our understanding of the origin of ultra high energy cosmic rays, the energy region between 1017 and 1019 eV is of crucial importance. Previous experiments have found indirect evidence that at these energies, the origin of cosmic rays changes from predominantly Galactic to extragalactic. In addition, weak evidence for an excess of cosmic rays from the direction of the Galactic center in a narrow energy band around 1018 eV has been claimed. However, so far there is no additional evidence supporting this scenario. Neither Galactic nor extragalactic sources have been unambiguously established. Given the importance of this energy range, there is a strong case for a dedicated experiment to study the EeV energy region with high precision. We describe the design and capabilities of a portable air fluorescence detector for stereo viewing of air showers at sub-EeV energies. located at a site on the southern hemisphere, the instrument proposed here will provide an accurate map of the Galactic center region, long susp ected to harb or one or several sources of ultra high energy cosmic rays. It will provide information on the chemical composition of any observed excess, and measure the energy spectrum in the region of the second knee.