An Air shower is an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray (i.e. one of extraterrestrial origin) enters our atmosphere. The term cascade means that the incident particle, which could be a proton, a nucleus, an electron, or (rarely) a positron strikes an atom in the air so as to produce many high energy ions (secondaries), which in turn create more, and so on. The original particle having arrived with high energy and hence velocity near the speed of light, the products of the collisions tend also to move generally downward, while to some extent spreading sidewise. The overall effect, when the energy of the primary is high enough, is to produce a widespread flash of light due to the Cerenkov effect, and to excitation of air molecules. This can be detected with arrays of mirrors and photocells. The actual arrival of the cascade of particles can also be detected in many cases, also generally with detectors based on the Cerenkov effect.
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