Ultrahigh-Energy Cosmic Rays: Results and Prospects

Observations of cosmic rays have been improved at all energies, both in terms of higher statistics and reduced systematics. As a result, the all-particle cosmic ray energy spectrum starts to exhibit more structures than could be seen previously. Most importantly, a second knee in the cosmic ray spectrum—dominated by heavy primaries—is reported just below 1017 eV. The light component, on the other hand, exhibits an ankle-like feature above 1017 eV and starts to dominate the flux at the ankle. The key question at the highest energies is about the origin of the flux suppression observed at energies above 5 · 1019 eV. Is this the long-awaited Greisen-Zatsepin-Kuzmin effect or the exhaustion of sources? The key to answering this question is again given by the still largely unknown mass composition at the highest energies. Data from different observatories do not quite agree, and common efforts have been started to settle that question. The high level of isotropy observed even at the highest energies starts to challenge a proton-dominated composition if extragalactic magnetic fields are on the order of a few nanogauss or more. We shall discuss the experimental and theoretical progress in the field and the prospects for the next decade.