Galactic cosmic ray

Galactic cosmic rays (GCRs) consist of those cosmic rays that enter the solar system from the outside. They are high-energy charged particles composed of protons, electrons, and fully ionized nuclei of light elements.

Origin

The magnetic fields of the Earth, the sun, and the galaxy itself tend to scramble the paths of the galactic cosmic rays, so that when one is detected, nothing can be inferred from the direction of its origin. Because many galactic cosmic rays are of extremely high energy, they must have originated in very energetic processes. Some are believed to have been accelerated by the shockwaves of supernovae. In the high-energy tail of the distribution, some galactic cosmic rays have energies so high that no known physical process could have created them.

Some of the isotopes observed in galactic cosmic rays have half-lives that are comparable to the time interval since their formation, and isotopic ratios can therefore carry some information about the amount of time that has passed since they were formed. In a few cases, there are isotopes in galactic cosmic rays which are unstable with respect to electron capture, but because they are fully stripped, they have not decayed in flight.

Observation

Most galactic cosmic rays have energies too low to penetrate the earth's atmosphere, and the radii of their helical trajectories in the earth's magnetic field tends to channel them to the poles; in this respect, these galactic cosmic rays are exactly like the charged particles that make up the solar wind. When they strike the atmosphere, they can create large showers of secondary particles, including exotic ones such as muons, and these secondary particles are what can be detected at the earth's surface.

Very high-energy cosmic rays can penetrate the earth's atmosphere, and the radii of their helical trajectories are thousands of kilometers, so they are not as effectively channeled by the earth's magnetic field.

As a radiation hazard

The Earth is naturally protected and the energy input of GCRs to the atmosphere is about 10^-9 of solar radiation - roughly the same as starlight. Jasper Kirkby; [http://arxiv.org/abs/0804.1938v1 Cosmic Rays And Climate] CERN-PH-EP/2008-005 26 March 2008 ]

However, Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. See Health threat from cosmic rays.

References

External links

* [http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html What Are Cosmic Rays?]
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* [http://news.bbc.co.uk/hi/english/sci/tech/newsid_1945000/1945504.stm 23 April, 2002, BBC News, Cosmic ray mystery 'solved']
* [http://news.bbc.co.uk/1/hi/sci/tech/3981619.stm 4 November, 2004, BBC News, Supernova produces cosmic rays]
* [http://www.srl.caltech.edu/ACE/ACENews/ACENews9.html "Clock Isotopes" in Cosmic Rays]
* [http://imagine.gsfc.nasa.gov/docs/features/topics/instrument_design/cosmic_design.html Cosmic Rays - Samples of matter]