Background image: The Aurora-Borealis, caused by cosmic rays striking the upper atmosphere
Cosmic rays are highly energetic particles travelling at a speed relative to the Earth close to the speed of light. One cannot identify one single source of cosmic rays. They are produced during violent events such as supernovae explosions, nuclear processes at the centre of galaxies or solar flares, and they are accelerated by shock waves of these explosions and more complex processes involving magnetic fields. Some cosmic rays are identified as solar energetic particles produced by energetic solar events; some come from interstellar space and called anomalous cosmic rays; others come from outside the solar system and called galactic cosmic rays.
The energy spectrum of cosmic rays can cover energies over 100 billion giga-electron volts (10¹¹ GeV). The most recent experiment covers the energy range from 1.6 x 10⁶ GeV to 10⁹ GeV, which is the interval where the transition from the rays in our galaxy to those produced outside is expected to occur. Because of these high energies cosmic rays can be very damaging to the human body, particularly outside the atmosphere.
Cosmic rays were discovered in 1912 by Victor Hess. At the time of discovery they were closely related to the study of electromagnetic radiation. However, although early instruments used in the detection of cosmic rays also responded to energetic gamma rays, the latter are not considered to be cosmic rays.