Cosmic ray

A high-energy particle or atomic nucleus traveling through space. When it strikes Earth’s atmosphere, it can generate a cascade of secondary particles.

Air shower: A cascade of secondary particles produced when a high-energy cosmic ray interacts with the atmosphere. This cascade is the atmospheric signature of a primary cosmic ray.
Solar energetic particle: A high-energy particle accelerated by solar flares or coronal mass ejections. The Sun supplies a variable, relatively low-energy component of cosmic radiation.
Primary cosmic ray: A cosmic-ray particle arriving from space before interacting with matter near Earth. Its identity and energy are the properties experiments try to infer from shower data.
Cosmic-ray muon: A penetrating elementary particle produced in atmospheric showers initiated by cosmic rays. Muons reach the ground in abundance and enable measurements beneath otherwise opaque structures.
Forbush decrease: A temporary reduction in galactic cosmic-ray intensity near Earth following an interplanetary shock or coronal mass ejection. It shows how solar disturbances can shield Earth from some incoming galactic particles.
Extensive air shower: A large atmospheric particle cascade initiated by an energetic cosmic ray and spread across a broad area. The highest-energy primaries create showers detectable across extensive ground arrays.
Supernova remnant: The expanding debris and shock structures left after a supernova explosion. Shock acceleration in remnants is a leading explanation for many Galactic cosmic rays.
Secondary cosmic ray: A particle created when a primary cosmic ray interacts with matter or radiation. Atmospheric collisions transform primaries into the particles commonly detected at ground level.
Muon tomography: An imaging technique that uses the absorption or deflection of cosmic-ray muons to map structures. Naturally occurring shower muons can reveal density inside volcanoes, reactors, and other large objects.
Solar modulation: The variation in galactic cosmic-ray intensity near Earth caused by the changing solar wind and magnetic field. It alters the low-energy cosmic-ray flux measured through the solar cycle.

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