Extending the Pierre Auger Observatory to explore the ankle of the cosmic ray spectrum Original Research Article

There are currently two enhancements of the Pierre Auger Observatory under construction or taking first data. AMIGA extends the surface detector by a denser array of water Cherenkov stations each accompanied by a muon counter. Furthermore, the fluorescence detector is extended by three telescopes, called HEAT, with a field of view chosen to observe lower atmospheric depths. Together they will lower the energy threshold of the Observatory by an order of magnitude from 1018 eV to about 1017 eV, and make it possible to study the interesting energy region around the second knee and the ankle with high precision and statistic. As the hybrid technique is best suited for determining the mass composition of cosmic rays, these enhancements will help to enlighten the questions around the transition from galactic to extragalactic cosmic rays.