Solar cosmic ray

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Solar cosmic rays are particle radiation that originate from the Sun. Most are made of protons with relatively low in energy (10-100 keV; 1.6 - 16 fJ/particle). The average composition is similar to that of the Sun itself.

The name solar cosmic ray itself is a misnomer since 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 higher and higher energies.^{[citation needed]} 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 the solar and cosmic 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.^{[citation needed]} 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.^{[citation needed]}