Editing Cosmic ray (section)

==Types==
Cosmic rays can be divided into two types:
* '''galactic cosmic rays''' ('''GCR''') and '''extragalactic cosmic rays''', i.e., high-energy particles originating outside the solar system, and
* '''[[solar energetic particles]]''', high-energy particles (predominantly protons) emitted by the sun, primarily in [[solar particle event|solar eruptions]].
However, the term "cosmic ray" is often used to refer to only the extrasolar flux.

[[File:Atmospheric Collision.svg|right|400px|thumb|Primary cosmic particle collides with a molecule of atmosphere, creating an air shower.]]
Cosmic rays originate as primary cosmic rays, which are those originally produced in various astrophysical processes. Primary cosmic rays are composed mainly of protons and alpha particles (99%), with a small amount of heavier nuclei (≈1%) and an extremely minute proportion of positrons and antiprotons.<ref name=goddard-2012/> Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, [[hadron]]s, and [[lepton]]s, such as [[electron]]s, positrons, muons, and [[pion]]s. The latter three of these were first detected in cosmic rays.

===Primary cosmic rays===
Primary cosmic rays mostly originate from outside the [[Solar System]] and sometimes even outside the [[Milky Way]]. When they interact with Earth's atmosphere, they are converted to secondary particles. The mass ratio of helium to hydrogen nuclei, 28%, is similar to the primordial [[elemental abundance]] ratio of these elements, 24%.<ref>{{cite web|author=Mewaldt, Richard A.|year=1996|title=Cosmic Rays|publisher=[[California Institute of Technology]]|url=http://www.srl.caltech.edu/personnel/dick/cos_encyc.html|access-date=26 December 2012|archive-date=30 August 2009|archive-url=https://web.archive.org/web/20090830191145/http://www.srl.caltech.edu/personnel/dick/cos_encyc.html|url-status=dead}}</ref> The remaining fraction is made up of the other heavier nuclei that are typical nucleosynthesis end products, primarily [[lithium]], [[beryllium]], and [[boron]]. These nuclei appear in cosmic rays in  greater abundance (≈1%) than in the solar atmosphere, where they are only about 10{{sup|−3}} as abundant (by number) as [[helium]]. Cosmic rays composed of charged nuclei heavier than helium are called [[HZE ions]]. Due to the high charge and heavy nature of HZE ions, their contribution to an astronaut's [[radiation dose]] in space is significant even though they are relatively scarce.

This abundance difference is a result of the way in which secondary cosmic rays are formed. Carbon and oxygen nuclei collide with interstellar matter to form [[lithium]], [[beryllium]], and [[boron]], an example of [[cosmic ray spallation]]. Spallation is also responsible for the abundances of [[scandium]], [[titanium]], [[vanadium]], and [[manganese]] [[ion]]s in cosmic rays produced by collisions of iron and nickel nuclei with [[interstellar medium|interstellar matter]].<ref>{{cite journal|title=The relative abundances of the elements scandium to manganese in relativistic cosmic rays and the possible radioactive decay of manganese 54|author1=Koch, L.|author2=Engelmann, J. J.|author3=Goret, P.|author4=Juliusson, E.|author5=Petrou, N.|author6=Rio, Y.|author7=Soutoul, A.|author8=Byrnak, B.|author9=Lund, N.|author10=Peters, B.|journal=Astronomy and Astrophysics|date=October 1981|volume=102|issue=11|bibcode=1981A&A...102L...9K|page=L9}}</ref>

At high energies the composition changes and heavier nuclei have larger abundances in some energy ranges. Current experiments aim at more accurate measurements of the composition at high energies.

====Primary cosmic ray antimatter====
{{see also|Alpha Magnetic Spectrometer}}
Satellite experiments have found evidence of positrons and a few antiprotons in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays. These do not appear to be the products of large amounts of antimatter from the Big Bang, or indeed complex antimatter in the universe. Rather, they appear to consist of only these two elementary particles, newly made in energetic processes.

Preliminary results from the presently operating [[Alpha Magnetic Spectrometer]] (''AMS-02'') on board the [[International Space Station]] show that positrons in the cosmic rays arrive with no directionality. In September 2014, new results with almost twice as much data were presented in a talk at CERN and published in Physical Review Letters.<ref>{{cite journal|first1=L.|last1=Accardo|display-authors=etal|collaboration=AMS Collaboration|title=High statistics measurement of the positron fraction in primary cosmic rays of 0.5–500&nbsp;GeV with the alpha magnetic spectrometer on the International Space Station|journal=Physical Review Letters|date=18 September 2014|volume=113|issue=12|page=121101|doi=10.1103/PhysRevLett.113.121101|pmid=25279616|url=http://ams.nasa.gov/Documents/AMS_Publications/PhysRevLett.113.121101.pdf |archive-url=https://web.archive.org/web/20141017131844/http://ams.nasa.gov/Documents/AMS_Publications/PhysRevLett.113.121101.pdf |archive-date=2014-10-17 |url-status=live|bibcode=2014PhRvL.113l1101A|doi-access=free}}</ref><ref>{{Cite journal|last1=Schirber|first1=Michael|year=2014|title=Synopsis: More dark matter hints from cosmic rays?|journal=Physical Review Letters|volume=113|issue=12|page=121102|doi=10.1103/PhysRevLett.113.121102|pmid=25279617|arxiv=1701.07305|bibcode=2014PhRvL.113l1102A|url=https://cds.cern.ch/record/1756487|hdl=1721.1/90426|s2cid=2585508}}</ref> A new measurement of positron fraction up to 500 GeV was reported, showing that positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of {{nowrap|275 ± 32 GeV}}. At higher energies, up to 500&nbsp;GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500&nbsp;GeV, but peaks at energies far higher than electron energies, which peak about 10&nbsp;GeV.<ref>{{cite web|title=New results from the Alpha Magnetic$Spectrometer on the International Space Station|url=http://ams.nasa.gov/Documents/AMS_Publications/ams_new_results_-_18.09.2014.pdf |archive-url=https://web.archive.org/web/20140923222913/http://ams.nasa.gov/Documents/AMS_Publications/ams_new_results_-_18.09.2014.pdf |archive-date=2014-09-23 |url-status=live|website=AMS-02 at NASA|access-date=21 September 2014}}</ref> These results on interpretation have been suggested to be due to positron production in annihilation events of massive [[dark matter]] particles.<ref>{{Cite journal|last1=Aguilar|first1=M.|last2=Alberti|first2=G.|last3=Alpat|first3=B.|last4=Alvino|first4=A.|last5=Ambrosi|first5=G.|last6=Andeen|first6=K.|last7=Anderhub|first7=H.|last8=Arruda|first8=L.|last9=Azzarello|first9=P.|last10=Bachlechner|first10=A.|last11=Barao|first11=F.|last12=Baret|first12=B.|last13=Barrau|first13=A.|last14=Barrin|first14=L.|last15=Bartoloni|first15=A.|last16=Basara|first16=L.|last17=Basili|first17=A.|last18=Batalha|first18=L.|last19=Bates|first19=J.|last20=Battiston|first20=R.|last21=Bazo|first21=J.|last22=Becker|first22=R.|last23=Becker|first23=U.|last24=Behlmann|first24=M.|last25=Beischer|first25=B.|last26=Berdugo|first26=J.|last27=Berges|first27=P.|last28=Bertucci|first28=B.|last29=Bigongiari|first29=G.|last30=Biland|first30=A.|display-authors=6|year=2013|title=First result from the Alpha Magnetic Spectrometer on the International Space Station: Precision measurement of the positron fraction in primary cosmic rays of 0.5–350&nbsp;GeV|journal=Physical Review Letters|volume=110|issue=14|pages=141102 |pmid=25166975 |bibcode=2013PhRvL.110n1102A|doi=10.1103/PhysRevLett.110.141102|url=https://boa.unimib.it/bitstream/10281/44680/1/2013_PhysRevLett.110.141102_positron_fraction.pdf |archive-url=https://web.archive.org/web/20170813134328/https://boa.unimib.it/bitstream/10281/44680/1/2013_PhysRevLett.110.141102_positron_fraction.pdf |archive-date=2017-08-13 |url-status=live|doi-access=free}}</ref>

Cosmic ray antiprotons also have a much higher average energy than their normal-matter counterparts (protons). They arrive at Earth with a characteristic energy maximum of 2&nbsp;GeV, indicating their production in a fundamentally different process from cosmic ray protons, which on average have only one-sixth of the energy.<ref>{{cite journal|title=Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere|author1=Moskalenko, I.V.|author2=Strong, A.W.|author3=Ormes, J.F.|author4=Potgieter, M.S.|date=January 2002|journal=The Astrophysical Journal|volume=565|issue=1|pages=280–296|doi=10.1086/324402|arxiv=astro-ph/0106567|bibcode=2002ApJ...565..280M|s2cid=5863020}}</ref>

There is no evidence of complex antimatter atomic nuclei, such as [[antihelium]] nuclei (i.e., anti-alpha particles), in cosmic rays. These are actively being searched for. A prototype of the ''AMS-02'' designated ''AMS-01'', was flown into space aboard the {{OV|103}} on [[STS-91]] in June 1998. By not detecting any antihelium at all, the ''AMS-01'' established an upper limit of {{nowrap|1.1 × 10<sup>−6</sup>}} for the antihelium to helium [[flux]] ratio.<ref>{{cite journal|collaboration=AMS Collaboration|last1=Aguilar|first1=M.|last2=Alcaraz|first2=J.|last3=Allaby|first3=J.|last4=Alpat|first4=B.|last5=Ambrosi|first5=G.|last6=Anderhub|first6=H.|last7=Ao|first7=L.|last8=Arefiev|first8=A.|display-authors=6|date=August 2002|title=The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part&nbsp;I – Results from the test flight on the space shuttle|journal=Physics Reports|volume=366|issue=6|pages=331–405|bibcode=2002PhR...366..331A|doi=10.1016/S0370-1573(02)00013-3|hdl=2078.1/72661|s2cid=122726107 }}</ref>
{{multiple image||align=left|direction=vertical|header=The moon in cosmic rays|height=200|image1=Moon's shadow in muons.gif|alt1=The moon's muon shadow|caption1=The [[Moon]]'s cosmic ray shadow, as seen in secondary muons detected 700&nbsp;m below ground, at the [[Soudan 2|Soudan&nbsp;2]] detector|image2=Moon egret.jpg|alt2=The moon as seen in gamma rays|caption2=The Moon as seen by the [[Compton Gamma Ray Observatory]], in gamma rays with energies greater than 20&nbsp;MeV. These are produced by cosmic ray bombardment on its surface.<ref>{{cite web|date=1 August 2005|title=EGRET detection of gamma rays from the Moon|url=http://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/gammoon.html |publisher=NASA|department=[[GSFC]]|access-date=11 February 2010}}</ref>}}

===Secondary cosmic rays===
When cosmic rays enter the [[Earth's atmosphere]], they collide with [[atom]]s and [[molecule]]s, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including [[x-rays]], protons, alpha particles, pions, muons, electrons, neutrinos, and [[neutron]]s.<ref>{{cite book|last=Morison|first=Ian|title=Introduction to Astronomy and Cosmology|year=2008|publisher=John Wiley & Sons|isbn=978-0-470-03333-3|page=198|bibcode=2008iac..book.....M}}</ref> All of the secondary particles produced by the collision continue onward on paths within about one degree of the primary particle's original path.

Typical particles produced in such collisions are neutrons and charged [[meson]]s such as positive or negative pions and [[kaon]]s. Some of these subsequently decay into muons and neutrinos, which are able to reach the surface of the Earth. Some high-energy muons even penetrate for some distance into shallow mines, and most neutrinos traverse the Earth without further interaction. Others decay into photons, subsequently producing electromagnetic cascades. Hence, next to photons, electrons and positrons usually dominate in air showers. These particles as well as muons can be easily detected by many types of particle detectors, such as [[cloud chamber]]s, [[bubble chamber]]s, [[High Altitude Water Cherenkov Experiment|water-Cherenkov]], or [[scintillation (physics)|scintillation]] detectors. The observation of a secondary shower of particles in multiple detectors at the same time is an indication that all of the particles came from that event.

Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high-energy gamma ray emissions by gamma-ray telescope. These are distinguished from radioactive decay processes by their higher energies above about 10&nbsp;MeV.

===Cosmic-ray flux===
[[File:SpaceEnvironmentOverview From 19830101.jpg|thumb|right|400px|An overview of the space environment shows the relationship between the solar activity and galactic cosmic rays.<ref>{{cite web|title=Extreme Space Weather Events|publisher=[[National Geophysical Data Center]]|url=http://sxi.ngdc.noaa.gov/sxi_greatest.html|access-date=19 April 2012|archive-date=22 May 2012|archive-url=https://web.archive.org/web/20120522031032/http://sxi.ngdc.noaa.gov/sxi_greatest.html|url-status=dead}}</ref>]]
The flux of incoming cosmic rays at the upper atmosphere is dependent on the [[solar wind]], the [[Earth's magnetic field]], and the energy of the cosmic rays. At distances of ≈94&nbsp;[[astronomical unit|AU]] from the Sun, the solar wind undergoes a transition, called the [[termination shock]], from supersonic to subsonic speeds. The region between the termination shock and the [[heliopause (astronomy)|heliopause]] acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤&nbsp;1&nbsp;GeV) by about&nbsp;90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.

In addition, the Earth's magnetic field acts to deflect cosmic rays from its surface, giving rise to the observation that the flux is apparently dependent on [[latitude]], [[longitude]], and [[azimuth|azimuth angle]].

The combined effects of all of the factors mentioned contribute to the flux of cosmic rays at Earth's surface. The following table of participial frequencies reach the planet<ref>{{cite web|url=http://www.auger.org/cosmic_rays/faq.html#how_many|title=How many?|series=Cosmic rays|publisher=Pierre Auger Observatory|website=Auger.org|access-date=17 August 2012|url-status=dead|archive-url=https://web.archive.org/web/20121012090629/http://www.auger.org/cosmic_rays/faq.html#how_many|archive-date=12 October 2012}}</ref> and are inferred from lower-energy radiation reaching the ground.<ref>{{cite web|url=https://www.auger.org/index.php/cosmic-rays/cosmic-ray-mystery|title=The mystery of high-energy cosmic rays|publisher=Pierre Auger Observatory|website=Auger.org|access-date=15 July 2015|archive-date=8 March 2021|archive-url=https://web.archive.org/web/20210308042139/https://www.auger.org/index.php/cosmic-rays/cosmic-ray-mystery|url-status=dead}}</ref>

::{| class="wikitable"
|+Relative particle energies and rates of cosmic rays
!scope="col"| Particle energy ([[Electronvolt|eV]])
!scope="col"| Particle rate (m{{sup|−2}}s{{sup|−1}})
|-
!scope="row"| {{val|1|e=9}} ([[GeV]])
| {{val|1|e=4}}
|-
!scope="row"| {{val|1|e=12}} ([[TeV]])
| 1 
|-
!scope="row"| {{val|1|e=16}} (10&nbsp;[[PeV]])
| {{val|1|e=-7}}
|-
!scope="row"| {{val|1|e=20}} (100&nbsp;[[EeV]])
| {{val|1|e=-15}}
|}

In the past, it was believed that the cosmic ray flux remained fairly constant over time. However, recent research suggests one-and-a-half- to two-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.<ref>{{cite journal|first1=D.|last1=Lal|first2=A.J.T.|last2=Jull|first3=D.|last3=Pollard|first4=L.|last4=Vacher|year=2005|title=Evidence for large century time-scale changes in solar activity in the past 32&nbsp;Kyr, based on in-situ cosmogenic <sup>14</sup>C in ice at Summit, Greenland|journal=[[Earth and Planetary Science Letters]]|volume=234|issue=3–4|pages=335–349|doi=10.1016/j.epsl.2005.02.011|bibcode=2005E&PSL.234..335L}}</ref>

The magnitude of the energy of cosmic ray flux in interstellar space is very comparable to that of other deep space energies: cosmic ray energy density averages about one electron-volt per cubic centimetre of interstellar space, or ≈1&nbsp;eV/cm<sup>3</sup>, which is comparable to the energy density of visible starlight at 0.3&nbsp;eV/cm<sup>3</sup>, the [[galactic magnetic fields|galactic magnetic field]] energy density (assumed 3&nbsp;microgauss) which is ≈0.25&nbsp;eV/cm<sup>3</sup>, or the [[cosmic microwave background]] (CMB) radiation energy density at ≈0.25&nbsp;eV/cm<sup>3</sup>.<ref>{{cite book|author1=Castellina, Antonella|author2=Donato, Fiorenza|author2-link=Fiorenza Donato|year=2012|title=Planets, Stars, and Stellar Systems|chapter=Astrophysics of Galactic charged cosmic rays|editor1=Oswalt, T.D.|editor2=McLean, I.S.|editor3=Bond, H.E.|editor4=French, L.|editor5=Kalas, P.|editor6=Barstow, M.|editor7=Gilmore, G.F.|editor8=Keel, W.|publisher=Springer|isbn=978-90-481-8817-8|edition=1}}</ref>