Air shower (physics)

Air showers are extensive cascades of subatomic particles and ionized nuclei, produced in the atmosphere when a primary cosmic ray enters the atmosphere. When a particle of the cosmic radiation, which could be a proton, a nucleus, an electron, a photon, or (rarely) a positron, interacts with the nucleus of a molecule in the atmosphere, it produces a vast number of secondary particles, which make up the shower. In the first interactions of the cascade especially hadrons (mostly light mesons like pions and kaons) are produced and decay rapidly in the air, producing other particles and electromagnetic radiation, which are part of the shower components. Depending on the energy of the cosmic ray, the detectable size of the shower can reach several kilometers in diameter.

The absorbed ionizing radiation from cosmic radiation is largely from muons, neutrons, and electrons, with a dose rate that varies in different parts of the world and is based largely on the geomagnetic field, altitude, and solar cycle. Airline crews are exposed to more radiation from cosmic rays if they routinely work flight routes that take them close to the North or South pole at high altitudes, where the shielding by the geomagnetic field is minimal.

The air shower phenomenon was unknowingly discovered by Bruno Rossi in 1933 in a laboratory experiment. In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that cosmic-ray particles are of extremely high energies and interact with nuclei high up in the atmosphere, initiating a cascade of secondary interactions that produce extensive showers of subatomic particles. ^[1][2]

The most important experiments detecting extensive air showers today are the Telescope Array Project and the Pierre Auger Observatory. The latter is the largest observatory for cosmic rays ever built, operating with 4 fluorescence detector buildings and 1600 surface detector stations spanning an area of 3,000 km² in the Argentinean desert.