The cascade of secondary particles and photons produced when a primary cosmic ray enters the Earth’s atmosphere and collides with air molecules; also known as an air shower or an Auger shower. The secondary cosmic rays are initially pions. Neutral pions decay (disintegrate) into gamma rays, which in turn generate electrons and positrons by pair production. Deceleration of the electrons and positrons by the atmosphere produces more photons by bremsstrahlung radiation. The charged pions decay into muons which themselves decay into electrons and neutrinos. The primary cosmic ray and many of the secondaries may undergo further collisions, yielding more particles. The resulting shower can cover an area several kilometres wide at ground level, depending on the energy of the particle that caused it. For example, the shower generated from a single cosmic ray of energy 10¹⁹ eV will cover 10 square kilometres and contain 10 billion particles at ground level.