An Etymological Dictionary of Astronomy and Astrophysics

Extremely energetic atomic nuclei which travel through the Universe at practically the speed of light and strike the Earth from all direction.

Almost 90% of all the incoming → primary cosmic rays are → protons, about 9% are helium nuclei (→ alpha particles) and about 1% are → electrons (beta minus particles). Some cosmic rays come from the Sun (mainly due to → solar flares), most come from galactic → supernovae, and a few with the highest energy are suspected to originate from outside the → Milky Way.

As for their flux, about 1 charged particle per second per cm² impacts the Earth. The typical kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon, although there are some at higher energies. In fact, the cosmic ray with the highest energy has been measured above × 10²⁰ eV. These → ultra-high energy cosmic rays are suspected to be extragalactic, as there is no plausible mechanism of acceleration to these energies by a supernova, for example. Again, compare these energies to those of solar neutrinos that have only 0.26 MeV.

Cosmic rays may be divided into → primary cosmic rays and → secondary cosmic rays.

Their energy
ranges from 10⁹ to 10²⁰  → electron-volts.

See also: → cosmic; → ray; The term “ray” is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.