An atomic particle moving close to the speed of light, with extremely high energy (10⁸–10²⁰ eV). On collision with atoms in the Earth’s atmosphere, a primary cosmic ray produces showers of secondary cosmic rays. These secondary particles decay into electrons, positrons, and neutrinos. Deceleration of the electrons and positrons by the atmosphere produces a flash of light which can be observed from the ground with a special telescope, and gives information about the primary cosmic ray.