Without qualification, usually means the primary cosmic rays of extra-terrestrial origin that continually bombard the earth and consist mostly of high-energy protons, about 9% helium and heavier nuclei, a small percentage of electrons, and some gamma rays. The energies of cosmic rays are well in excess of billions of electron volts. Secondary cosmic rays result from interactions between primary rays and atoms in the earth's atmosphere. Most cosmic rays probably originate from the Milky Way galaxy, but a small fraction come from the sun as evidenced by diurnal variations in the cosmic ray flux.