Solar cosmic ray

From Academic Kids

Solar cosmic rays are cosmic rays that originate from the Sun. Most are made of protons; these rays are relatively low in energy (10-100 keV). The average composition is similar to that of the Sun itself. The name solar cosmic ray itself is a misnomer, but it has stuck. High energy (Mev and above) cosmic rays come mainly from outside the solar system, while the particles in the solar case are energized near the Sun's surface by the action of magnetic fields. 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 higher and higher energies. Until the mid 1960's 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 (sic) in the galactic cosmic rays, called a "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 the solar and galactic particles, mainly in that the galactic ones 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 are assumed to result from the spallation (fragmentation) of heavy nuclei due to collisions in transit from the distant sources to the solar system.

External links

Time variations: [1] (http://collections.ic.gc.ca/simply_science/scirep5i.htm)

A broad, rather detailed and technical survey: [2] (http://www.int.washington.edu/PHYS554/winter_2004/chapter8_04.pdf)

Composition of Galactic cosmic rays: [3] (http://dphs10.saclay.cea.fr/Sap/Activites/RapportActivites/Science/Compact/compact11_gb.php)

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