Past, Present and Future of UHECR Observations

Understand

Great advances have been made in the study of ultra-high energy cosmic rays (UHECR) in the past two decades.

• These include the discovery of the spectral cut-off near 5 x 10^19 eV and complex structure at lower energies, as well as increasingly precise information about the composition of cosmic rays as a function of energy.
• Important improvements in techniques, including extensive surface detector arrays and high resolution air fluorescence detectors, have been instrumental in facilitating this progress.
• We discuss the status of the field, including the open questions about the nature of spectral structure, systematic issues related to our understanding of composition, and emerging evidence for anisotropy at the highest energies.