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Cosmic rays (CR) are not really rays at all, but particles. They are ionized atoms ranging from a single proton up to an iron nucleus and beyond, but being typically protons and alpha particles. They originate from space, being produced by a number of different sources, such as the Sun, other stars, and more exotic objects, such as supernova and their remnants, neutron stars and black holes, as well as active galactic nuclei and radio galaxies. Cosmic ray particles are travelling very close to the speed of light, and the most energetic particle ever observed had about 20 Joules of kinetic energy (equivalent to the energy of a fast shot ball). The number density of CR integrated over energy (> 100 MeV/nucleus) is about 10-10 cm-3 in the vicinity of the Earth. The total energy density of CR particles is about 1 eVcm-3.

There are three different types of cosmic rays (e.g., Mewaldt et al., 1994):

The acceleration of the cosmic rays to observed energies is still partly an open question, especially at the ultra high energy range. Two mechanisms that seem to play a role are the Fermi acceleration (suggested first by Enrico Fermi, 1949) and magnetic pumping (suggested by Hans Alfven, 1963).

The Earth's atmosphere protects us from being exposed to these particles. As a cosmic ray enters the atmosphere, it will collide with a particle in there (usually a nitrogen or oxygen molecule), or interact with molecules, exciting them and thus causing an outer space influence upon the Earth's environment. Cosmic rays are creating the same, harmfull effects in near space around the Earth as are the radiation belts and, in addition, affect radio communication in polar regions (see space weather). They can even affect the global climate system; see space climate.


  • Mewaldt, R. A., A. C. Cummings, and E. C. Stone, Anomalous cosmic rays: Interstellar interlopers in the heliosphere and magnetosphere, EOS, 75, Number 16, 1994.

See also Wikipedia on cosmic rays.

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