What happens when a cosmic ray enters Earth’s atmosphere?

When a cosmic ray—a high-speed particle such as an atomic nucleus or an electron—reaches the outer edge of Earth’s atmosphere, it is referred to as a "primary" particle. These primaries are not directly observed at the surface because they collide with atmospheric nuclei upon entry.12

These collisions trigger a cascade of events, giving rise to "secondaries". Some of these secondary particles are fragments of the original colliding nuclei, such as neutrons, while others are short-lived particles created from the energy of the collisions, including muons. These secondary nuclei may subsequently undergo their own collisions.23

In the case of very high-energy cosmic rays, these interactions produce extensive air showers (EASs) that can consist of billions of secondary particles, which may be detected at ground level over large areas. It is these secondary particles that are ultimately observed at sea level.32