Dear Colleagues,

The study of Ultra High Energy Cosmic Rays (E > 10¹⁸ eV) faces two great challenges: the extreme low fluxes; and our limited understanding of particle interaction properties at such high energies, which limits the interpretation of the observed air showers in terms of the composition of the primary cosmic ray. The composition spectrum of cosmic rays is critical to interpreting the features observed in the energy spectrum and to interpreting the observed anisotropy in their arrival directions.

The Pierre Auger Observatory (in the Southern Hemisphere) and the Telescope Array and Yakutsk Observatories (in the Northern Hemisphere) have provided most of the Ultra-High-Energy Cosmic Ray data. The Pierre Auger Observatory, with the largest collection area (3000 km²) and after more than 15 years of collecting data, has measured the energy spectrum, established that the arrival directions are anisotropic at the 4% level, and suggested that the cosmic ray composition is not dominated by protons at the highest energies. It has also provided evidence that current high-energy hadronic interaction models are predicting a lesser amount of muons than the ones recorded in air showers. Currently, the Pierre Auger Collaboration is upgrading its ground array. Each ground station will have a scintillation detector and a radio antenna that will operate together with the Water Cherenkov detector. The additional detectors will help to provide better measurements of the different components of the air shower (e.g., electromagnetic and muonic components), which should improve the interpretation in terms of the mass composition and improve our understanding of high-energy hadronic interactions.

The Telescope Array has measured an energy spectrum with similar features to the Auger one. However, there are some differences at the highest energies. More statistics are necessary; therefore, the Telescope Array is expanding its collection area (TA x 4), and its collection area will be similar to that of the Pierre Auger Collaboration.

This Special Issue aims to provide a series of reviews of the most recent measurements of the energy spectrum, mass composition spectrum, and arrival direction anisotropies and studies with air shower measurements regarding the validity of current high-energy hadronic interaction models. We also invite contributions covering other important issues, such as: astrophysical interpretations based on the most recent measurements of the energy spectrum, mass composition, and arrival direction anisotropies; the next generation of cosmic ray detectors for covering the energy range beyond 10^19.5 eV; and synergy between ultra-high-energy cosmic ray detectors, gamma-ray detectors, and neutrino ground array detectors for addressing the muon problem in hadronic interaction models and for producing a complete picture of cosmic rays over a wide range of energies.

Dr. Jose Alfredo Bellido
Guest Editor

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• high-energy cosmic rays
• astroparticle physics
• high-energy hadronic interactions
• high-energy cosmic ray detectors.