Cosmic rays are high-energy particles from space. In the atmosphere it produces cascades of billions of secondary particles (85% of protons, about 10% helium nuclei, about 2% of electrons, 1-2% are heavy nuclei, and less than 1% are antiparticles).
Cosmic rays can be measured directly using balloon and satellite based detectors up to PeV energies and ground based arrays detected extensive air-showers produced by high-energy cosmic rays up to EeV. It is still not clear how and where nature accelerates these cosmic particles. While the highest energies originate from other galaxies, it is still unclear region of transition from galactic to extragalactic cosmic ray sources. Also understanding the mass composition of cosmic rays is one of the most urgent questions in astrophysics.
Today ground based arrays for extensive air-showers detection are different: ground or underground detectors of secondary particles, detectors of electromagnetic radiation, in particular air-fluorescence and air-Cherenkov light at optical and ultraviolet frequencies, and radio emission.