Nature of the highest energy cosmic rays

Ultrahigh energy $\gamma$ rays produce electron-positron pairs in interactions on the geomagnetic field. The pair electrons suffer magnetic bremsstrahlung and the energy of the primary $\gamma$ ray is shared by a bunch of lower energy secondaries. These processes reflect the structure of the geomagnetic field and cause experimentally observable effects. The study of these effects with future giant air shower arrays can identify the nature of the highest energy cosmic rays as either $\gamma$ rays or nuclei.