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's cosmic ray shadow, as seen in secondary muons detected 700m below ground, at the Soudan 2 detector]] Cosmic rays are energetic particles originating from space that impinge on Earth's Atmosphere . Almost 90% of all the incoming cosmic ray particles are Proton s, about 9% are Helium nuclei ( Alpha Particle s) and about 1% are Electron s. Note that the term "ray" is a misnomer, as cosmic ray particles arrive individually, not in the form of a ray or beam of particles. See '' Wave-particle Duality ''. The Kinetic Energies of cosmic ray particles span over fourteen orders of magnitude, with the Flux of cosmic rays on Earth's surface falling approximately as the inverse-cube of the energy. The wide variety of particle energies reflects the wide variety of sources. Cosmic rays originate from energetic processes on the Sun all the way to the farthest reaches of the visible Universe . Cosmic rays can have energies of over 1020 EV , far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. (The article on Ultra-high-energy Cosmic Ray s describes the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s.) There has been interest in investigating cosmic rays of even greater energies.Luis Anchordoqui, Thomas Paul, Stephen Reucroft, John Swain. ''Ultrahigh Energy Cosmic Rays: The state of the art before the Auger Observatory''. (2002) arxiv:hep-ph/0206072 COSMIC RAY SOURCES Most cosmic rays originate from extrasolar sources within our own galaxy such as rotating Neutron Star s, Supernovae , and Black Holes . However, the fact that some cosmic rays have extremely high energies provides evidence that at least some must be of extra-galactic origin (e.g. Radio Galaxies and Quasars ); the local galactic magnetic field would not be able to contain particles with such a high energy. The origin of cosmic rays with energies up to 1014 eV can be accounted for in terms of shock-wave acceleration in supernova shells. The origin of cosmic rays with energy greater than 1014 eV remains unknown; however, a large collaborative experiment at the Pierre Auger Observatory is underway to try to answer this question. Observations have shown that cosmic rays with an energy above 10 GeV (10 x 109 eV) approach the Earth’s surface isotropically (equally from all directions); it has been hypothesised that this is not due to an even distribution of cosmic ray sources, but instead is due to galactic magnetic fields causing cosmic rays to travel in spiral paths. This limits cosmic ray’s usefulness in positional Astronomy as they carry no information of their direction of origin. At energies below 10 GeV there is a directional dependence, due to the interaction of the charged component of the cosmic rays with the Earth's Magnetic Field . Solar cosmic rays Solar cosmic rays are cosmic rays that originate from the Sun , with relatively low energy (10-100 keV or 1.6 - 16 fJ per particle). The average composition is similar to that of the Sun itself. The name solar cosmic ray itself is a misnomer because the term cosmic implies that the rays are from the cosmos and not the solar system, but it has stuck. The misnomer arose because there is continuity in the energy spectra, i.e., the flux of particles as a function of their energy, because the low-energy solar cosmic rays fade more or less smoothly into the galactic ones as one looks at increasingly higher energies. Until the mid-1960s the energy distributions were generally averaged over long time intervals, which also obscured the difference. Later, it was found that the solar cosmic rays vary widely in their intensity and spectrum, increasing in strength after some solar events such as solar flares. Further, an increase in the intensity of solar cosmic rays is followed by a decrease in all other cosmic rays, called the Forbush Decrease after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind with its entrained magnetic field sweeping some of the galactic cosmic rays outwards, away from the Sun and Earth. The overall or average rate of Forbush decreases tends to follow the 11-year sunspot cycle, but individual events are tied to events on the Sun, as explained above. There are further differences between cosmic rays of solar and galactic origin, mainly in that the galactic cosmic rays show an enhancement of heavy elements such as Calcium , Iron and Gallium , as well as of cosmically rare light elements such as Lithium and Beryllium . The latter result from the Cosmic Ray Spallation (fragmentation) of heavy nuclei due to collisions in transit from the distant sources to the solar system. Galactic cosmic rays See '' Galactic Cosmic Ray ''. Extragalactic cosmic rays See '' Extragalactic Cosmic Ray ''. Ultra-high-energy cosmic rays See '' Ultra-high-energy Cosmic Ray ''. Anomalous cosmic rays Anomalous cosmic rays (ACRs) are cosmic rays with unexpectedly low energies. They are thought to be created near the edge of our solar system, in the Heliosheath , the border region between the Heliosphere and the Interstellar Medium . When electrically neutral atoms are able to enter the heliosheath (being unaffected by its magnetic fields) subsequently become ionized, they are thought to be accelerated into low-energy cosmic rays by the Solar Wind 's Termination Shock which marks the inner edge of the heliosheath. It is also possible that high energy Galactic Cosmic Ray s which hit the Shock Front of the solar wind near the Heliopause might be decelerated, resulting in their transformation into lower-energy anomalous cosmic rays. The '') if this is typical of the termination shock (requiring a major rethink of the origin of ACRs), or a localised feature of that part of the termination shock that ''Voyager 1'' passed through. '' Voyager 2 '' is expected to cross the termination shock during or after 2008 , which will provide more data. COMPOSITION Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact with . This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termed Cosmic Ray Spallation ), into lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B falls off somewhat steeper than that of carbon or oxygen, indicating that less Cosmic Ray Spallation occurs for the higher energy nuclei presumably due to their escape from the Galactic Magnetic Field . Spallation is also responsible for the abundances of Sc, Ti, V and Mn elements in cosmic rays, which are produced by collisions of Fe and Ni nuclei with Interstellar Matter ; see Environmental Radioactivity#Natural s. In the past, it was believed that the cosmic ray Flux has remained fairly constant over time. Recent research has, however, produced evidence for 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years. |
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