Cosmic ray particles hit the Earth's atmosphere at the rate of about 1000 per square meter per second. They are ionized nuclei - about 90% protons, 9% alpha particles and the rest heavier nuclei - and they are distinguished by their high energies. Most cosmic rays are relativistic, having energies comparable to or somewhat greater than their masses. A very few of them have ultrarelativistic energies extending up to 10²⁰eV (about 20 joules), eleven orders of magnitude greater than the equivalent rest mass energy of a proton. The fundamental question of cosmic ray physics is, "Where do they come from?" and in particular, "How are they accelerated to such high energies?"

The answer to the question of the origin of cosmic rays is not yet fully known. It is clear, however , that nearly all of them come from outside the solar system, but from within the galaxy. The relatively few particles of solar origin are characterized by temporal association with violent events on the sun and consequently by a rapid variability. In contrast, the bulk of cosmic rays show an anticorrelation with solar activity, being more effectively excluded from the solar neighborhood during periods when the expanding, magnetized plasma from the sun - the solar wind - is most intense. The very highest energy cosmic rays have gyroradii in typical galactic magnetic fields that are larger than the size of the galaxy. These may be of extragalactic origin.