View source for Cosmic ray

{{short description|High-energy particle, mainly originating outside the Solar System}}
{{Redirect|Cosmic radiation|some other types of cosmic radiation|Cosmic background radiation|and|Cosmic background (disambiguation)|the film|Cosmic Ray (film)}}
{{Use dmy dates|date=September 2019}}
{{Use American English|date=March 2021}}

[[File:Cosmic ray flux versus particle energy.svg|360px|thumb|Cosmic [[flux]] versus particle energy at the top of Earth's atmosphere]]
[[File:Hard-component-muon-868x1024.png|thumb|right|Left image: cosmic ray muon passing through a cloud chamber undergoes scattering by a small angle in the middle metal plate and leaves the chamber. Right image: cosmic ray muon losing considerable energy after passing through the plate as indicated by the increased curvature of the track in a magnetic field.]]

'''Cosmic rays''' or '''astroparticles''' are [[high-energy particle]]s or clusters of particles (primarily represented by [[proton]]s or [[atomic nuclei]]) that move through space at nearly the [[speed of light]]. They originate from the [[Sun]], from outside of the [[Solar System]] in the [[Milky Way]],<ref>
{{cite book|last=Sharma|first=Shatendra|title=Atomic and Nuclear Physics|date=2008|publisher=Pearson Education India|isbn=978-81-317-1924-4|page=478}}
</ref> and from distant galaxies.<ref>{{cite news|title=Detecting cosmic rays from a galaxy far, far away|date=21 September 2017|website=Science Daily|url=https://www.sciencedaily.com/releases/2017/09/170921141257.htm|access-date=26 December 2017}}
</ref> Upon impact with [[Atmosphere of Earth|Earth's atmosphere]], cosmic rays produce [[air shower (physics)|showers of secondary particles]], some of which reach the [[Earth's surface|surface]], although the bulk are [[Deflection (physics)|deflected]] off into space by the [[Earth's magnetic field|magnetosphere]] or the [[heliosphere]].

Cosmic rays were discovered by [[Victor Francis Hess|Victor Hess]] in 1912 in balloon experiments, for which he was awarded the 1936 [[Nobel Prize in Physics]].<ref name=HessNobelPresSp/>

Direct measurement of cosmic rays, especially at lower energies, has been possible since the launch of the first satellites in the late 1950s. Particle detectors similar to those used in nuclear and high-energy physics are used on satellites and space probes for research into cosmic rays.<ref>{{cite book|editor=Cilek, Vaclav|year=2009|title=Earth System: History and Natural Variability|volume=I|page=165|chapter=Cosmic Influences on the Earth|publisher=Eolss Publishers|isbn=978-1-84826-104-4|chapter-url=https://www.eolss.net/ebooklib/bookinfo/earth-system-history-natural-variability.aspx}}
</ref> Data from the [[Fermi Gamma-ray Space Telescope|Fermi Space Telescope]] (2013)<ref name=ackermann-2013>{{cite journal|last1=Ackermann|first1=M.|last2=Ajello|first2=M.|last3=Allafort|first3=A.|last4=Baldini|first4=L.|last5=Ballet|first5=J.|last6=Barbiellini|first6=G.|last7=Baring|first7=M.G.|last8=Bastieri|first8=D.|last9=Bechtol|first9=K.|last10=Bellazzini|first10=R.|last11=Blandford|first11=R.D.|last12=Bloom|first12=E.D.|last13=Bonamente|first13=E.|last14=Borgland|first14=A.W.|last15=Bottacini|first15=E.|last16=Brandt|first16=T.J.|last17=Bregeon|first17=J.|last18=Brigida|first18=M.|last19=Bruel|first19=P.|last20=Buehler|first20=R.|last21=Busetto|first21=G.|last22=Buson|first22=S.|last23=Caliandro|first23=G.A.|last24=Cameron|first24=R.A.|last25=Caraveo|first25=P.A.|author25-link=Patrizia A. Caraveo|last26=Casandjian|first26=J.M.|last27=Cecchi|first27=C.|last28=Celik|first28=O.|last29=Charles|first29=E.|last30=Chaty|first30=S.|display-authors=6|date=15 February 2013|journal=[[Science (journal)|Science]]|volume=339|issue=6424|pages=807–811|title=Detection of the characteristic pion decay-signature in supernova remnants|doi=10.1126/science.1231160|arxiv=1302.3307|bibcode=2013Sci...339..807A|pmid=23413352|s2cid=29815601}}
</ref> have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the [[supernova]] explosions of stars.<ref name=pinholster-2013>{{cite press release|author=Pinholster, Ginger|date=13 February 2013| title=Evidence shows that cosmic rays come from exploding stars|publisher=[[American Association for the Advancement of Science]]|location=Washington, DC|url=http://www.aaas.org/news/releases/2013/0214_supernova_cosmicrays.shtml}}
</ref>{{better source needed|date=September 2023}}<!-- This is a press release. It would be better to directly cite the research it was based on. --> Based on observations of [[neutrino]]s and [[gamma ray]]s from [[blazar]] [[TXS 0506+056]] in 2018, [[active galactic nuclei]] also appear to produce cosmic rays.<ref name="nature">
{{cite journal
 |last1=Abramowski |first1=A.
 |collaboration=HESS Collaboration
 |year=2016
 |title= Acceleration of petaelectronvolt protons in the Galactic Centre
 |journal= [[Nature (journal)|Nature]]
 |volume=531 |issue= 7595 |pages= 476–479
 |doi= 10.1038/nature17147
 |arxiv = 1603.07730 |bibcode= 2016Natur.531..476H
 |pmid= 26982725 |s2cid= 4461199
}}
</ref><ref>{{cite journal|last=Aartsen|first=Mark|collaboration=IceCube Collaboration|date=12 July 2018|title=Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert|journal=Science|volume=361|issue=6398|pages=147–151|doi=10.1126/science.aat2890|issn=0036-8075|pmid=30002248 |bibcode=2018Sci...361..147I|arxiv=1807.08794|s2cid=133261745}}
</ref>

==Etymology==
The term ''ray'' (as in [[optical ray]]) seems to have arisen from an initial belief, due to their penetrating power, that cosmic rays were mostly [[electromagnetic radiation]].<ref>{{cite web|author=Christian, Eric|title=Are cosmic rays electromagnetic radiation?|publisher=NASA|url=http://helios.gsfc.nasa.gov/qa_cr.html#em|archive-url= https://web.archive.org/web/20000531200225/http://helios.gsfc.nasa.gov/qa_cr.html#em|archive-date=31 May 2000|access-date=11 December 2012}}
</ref> Nevertheless, following wider recognition of cosmic rays as being various high-energy particles with [[invariant mass|intrinsic mass]], the term "rays" is consistent with known particles such as [[cathode ray]]s, [[anode ray|canal rays]], [[alpha particle|alpha rays]], and [[beta particle|beta rays]]. Meanwhile "cosmic" ray [[photons]], which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as ''[[gamma ray]]s'' or ''[[X-ray]]s'', depending on their [[photon energy]].

==Composition==
Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the bare nuclei of common atoms (stripped of their electron shells), and about 1% are solitary electrons (that is, one type of [[beta particle]]). Of the nuclei, about 90% are simple [[proton]]s (i.e., hydrogen nuclei); 9% are [[alpha particle]]s, identical to helium nuclei; and 1% are the nuclei of heavier elements, called [[HZE ions]].<ref name=goddard-2012>{{cite web|title=What are cosmic rays?|publisher=NASA|department=Goddard Space Flight Center|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|access-date=31 October 2012|archive-url=https://web.archive.org/web/20121028154200/http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|archive-date=28 October 2012}}{{cite web|title=mirror copy, also archived|url=http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_GSFC/goddard/imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html|archive-url=https://web.archive.org/web/20160304080140/http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_GSFC/goddard/imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html|archive-date=4 March 2016}}
</ref> These fractions vary highly over the energy range of cosmic rays.<ref name=dembinski-2017>{{cite journal|author1=Dembinski, H.|display-authors=etal|year=2018|title=Data-driven model of the cosmic-ray flux and mass composition from 10 GeV to 10^11 GeV|journal=[[Proceedings of Science]]|volume=ICRC2017|page=533|doi=10.22323/1.301.0533|arxiv=1711.11432|s2cid=85540966 |doi-access=free }}</ref> A very small fraction are stable particles of [[antimatter]], such as [[positron]]s or [[antiproton]]s. The precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for [[Alpha particle#Anti-alpha particle|anti-alpha particles]] as of 2019<ref>{{cite web|title=Cosmic Rays|series=Science Toolbox|website=imagine.gsfc.nasa.gov|publisher=[[National Aeronautics and Space Administration]]|department=Goddard Space Flight Center|url=https://imagine.gsfc.nasa.gov/science/toolbox/cosmic_rays1.html|access-date=23 March 2019}}</ref> had found no unequivocal evidence.

Upon striking the atmosphere, cosmic rays violently burst atoms into other bits of matter, producing large amounts of [[pion]]s and [[muon]]s (produced from the decay of charged pions, which have a short half-life) as well as [[neutrino]]s.<ref name="Vox">{{cite web | last = Resnick | first = Brian | title = Extremely powerful cosmic rays are raining down on us. No one knows where they come from | publisher = [[Vox Media]]   | date = July 25, 2019 | url = https://www.vox.com/the-highlight/2019/7/16/17690740/cosmic-rays-universe-theory-science | access-date = 2022-12-14 }}</ref> The [[neutron]] composition of the particle cascade increases at lower elevations, reaching between 40% and 80% of the radiation at aircraft altitudes.<ref name="pmid32975102">{{cite journal | vauthors= Sovilj MP, Vuković B, Stanić D | title=Potential benefit of retrospective use of neutron monitors in improving ionising radiation exposure assessment on international flights: issues raised by neutron passive dosimeter measurements and EPCARD simulations during sudden changes in solar activity | journal=Arhiv Za Higijenu Rada I Toksikologiju | volume=71 | issue=2|  pages=152–157 | year=2020  | doi = 10.2478/aiht-2020-71-3403 | pmc=7968484 | pmid=32975102}}</ref>

Of secondary cosmic rays, the charged pions produced by primary cosmic rays in the atmosphere swiftly decay, emitting muons. Unlike pions, these muons do not interact strongly with matter, and can travel through the atmosphere to penetrate even below ground level. The rate of muons arriving at the surface of the Earth is such that about one per second passes through a volume the size of a person's head.<ref>CERN https://home.cern/science/physics/cosmic-rays-particles-outer-space</ref> Together with natural local radioactivity, these muons are a significant cause of the ground level atmospheric ionisation that first attracted the attention of scientists, leading to the eventual discovery of the primary cosmic rays arriving from beyond our atmosphere.

==Energy==
Cosmic rays attract great interest practically, due to the damage they inflict on microelectronics and life outside the protection of an atmosphere and magnetic field, and scientifically, because the energies of the most energetic [[ultra-high-energy cosmic ray]]s have been observed to approach {{nowrap|3 × {{10^|20}} eV&thinsp;}}<ref>{{cite news |author=Nerlich, Steve |date=12 June 2011 |title=Astronomy without a telescope – 'Oh-my-God' particles |website=Universe Today |url=http://www.universetoday.com/86490/astronomy-without-a-telescope-oh-my-god-particles/ |access-date=17 February 2013}}</ref> (This is slightly greater than 10&nbsp;million times the design energy of particles accelerated by the [[Large Hadron Collider]], {{convert|7|TeV|eV|abbr=~|lk=on}}.<ref>{{cite web |title=LHC: The guide |series=FAQ: Facts and figures |year=2021 |page=3 |publisher=[[European Organization for Nuclear Research]] (CERN) |department=[[Large Hadron Collider]] |url=https://home.web.cern.ch/resources/brochure/knowledge-sharing/lhc-facts-and-figures |access-date=9 October 2022}}</ref>) One can show that such enormous energies might be achieved by means of the [[centrifugal mechanism of acceleration]] in [[active galactic nuclei]]. At {{convert|50|J|GeV|abbr=~|lk=on}},<ref>{{cite magazine |author=Gaensler, Brian |date=November 2011 |title=Extreme speed |magazine=COSMOS |issue=41 |page= |url=http://ska.cosmosmagazine.com/features/print/5162/extreme-speed |archive-url=https://archive.today/20130407234845/http://ska.cosmosmagazine.com/features/print/5162/extreme-speed |archive-date=7 April 2013}}</ref> the highest-energy ultra-high-energy cosmic rays (such as the [[Oh-My-God particle|OMG particle]] recorded in 1991) have energies comparable to the kinetic energy of a {{convert|90|kph|mph|adj=on|abbr=~|lk=on}} baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies.<ref>{{cite journal |author1=Anchordoqui, L. |author2=Paul, T. |author3=Reucroft, S. |author4=Swain, J. |year=2003 |title=Ultrahigh energy cosmic rays: The state of the art before the Auger Observatory |journal=[[International Journal of Modern Physics A]] |volume=18 |issue=13 |pages=2229–2366 |doi=10.1142/S0217751X03013879 |arxiv=hep-ph/0206072 |bibcode=2003IJMPA..18.2229A |s2cid=119407673}}</ref> Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks at {{convert|300|MeV|J|abbr=~|lk=on}}.<ref>{{cite web |editor=Nave, Carl R. |title=Cosmic rays |website=HyperPhysics |publisher=[[Georgia State University]] |department=Physics and Astronomy Department |url=http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmic.html |access-date=17 February 2013}}</ref>

==History==
After the discovery of [[Radioactive decay|radioactivity]] by [[Henri Becquerel]] in 1896, it was generally believed that [[atmospheric electricity]], [[ionization]] of the [[Earth's atmosphere|air]], was caused only by [[radiation]] from radioactive elements in the ground or the radioactive gases or isotopes of [[radon]] they produce.<ref>{{cite book|last=Malley|first=Marjorie C.|date=25 August 2011|title=Radioactivity: A History of a Mysterious Science|pages=78–79|publisher=Oxford University Press|isbn=978-0-19-976641-3|url=https://books.google.com/books?id=yvtjtKSfX54C&pg=PA78}}</ref> Measurements of increasing ionization rates at increasing heights above the ground during the decade from 1900 to 1910 could be explained as due to absorption of the ionizing radiation by the intervening air.<ref>{{cite book|first=John|last=North|date=15 July 2008|title=Cosmos: An Illustrated History of Astronomy and Cosmology|page=686|publisher=University of Chicago Press|isbn=978-0-226-59441-5|url=https://books.google.com/books?id=qq8Luhs7rTUC&pg=PA686}}</ref>

===Discovery===
[[File:Pacini measurement.jpg|thumb|left|Pacini makes a measurement in 1910.|upright=0.8]]
In 1909, [[Theodor Wulf]] developed an [[electrometer]], a device to measure the rate of ion production inside a hermetically sealed container, and used it to show higher levels of radiation at the top of the [[Eiffel Tower]] than at its base.<ref>{{cite journal |last1=Wulf |first1=Theodor |title=Beobachtungen über die Strahlung hoher Durchdringungsfähigkeit auf dem Eiffelturm |journal=Physikalische Zeitschrift |date=1910 |volume=11 |pages=811–813 |url=https://babel.hathitrust.org/cgi/pt?id=njp.32101054770845&view=1up&seq=349&skin=2021 |trans-title=Observations of radiation of high penetration power at the Eiffel tower |language=German}}</ref> However, his paper published in ''[[Physikalische Zeitschrift]]'' was not widely accepted. In 1911, [[Domenico Pacini]] observed simultaneous variations of the rate of ionization over a lake, over the sea, and at a depth of 3 metres from the surface. Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth.<ref>{{cite journal|author=Pacini, D.|year=1912|title=La radiazione penetrante alla superficie ed in seno alle acque|journal=[[Il Nuovo Cimento]]|volume=3|issue=1|pages=93–100|doi=10.1007/BF02957440|arxiv=1002.1810|bibcode=1912NCim....3...93P|s2cid=118487938}}: Translated with commentary in
{{cite journal|author=de Angelis, A.|year=2010|title=Penetrating Radiation at the Surface of and in Water|journal=[[Il Nuovo Cimento]]|volume=3|issue=1|pages=93–100|arxiv=1002.1810|bibcode=1912NCim....3...93P|doi=10.1007/BF02957440|s2cid=118487938}}</ref>

In 1912, [[Victor Francis Hess|Victor Hess]] carried three enhanced-accuracy Wulf electrometers<ref name=HessNobelPresSp>{{cite web|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/press.html|title=Nobel Prize in Physics 1936 – Presentation Speech|publisher=Nobelprize.org|date=10 December 1936|access-date=27 February 2013}}</ref> to an altitude of 5,300&nbsp;metres in a [[hot air balloon|free balloon]] flight. He found the ionization rate increased to twice the rate at ground level.<ref name=HessNobelPresSp/> Hess ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes.<ref name=HessNobelPresSp/> He concluded that "The results of the observations seem most likely to be explained by the assumption that radiation of very high penetrating power enters from above into our atmosphere."<ref>{{cite journal|author=Hess, V.F. |author-link=Victor Francis Hess |year=1912 |title=Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten |trans-title=On observations of penetrating radiation during seven free balloon flights |language=de |journal=Physikalische Zeitschrift |volume=13 |pages=1084–1091 |arxiv=1808.02927}}</ref> In 1913–1914, [[Werner Kolhörster]] confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9&nbsp;km.<ref>{{cite journal |last1=Kolhörster |first1=Werner |title=Messungen der durchdringenden Strahlung im Freiballon in größeren Höhen |journal=Physikalische Zeitschrift |date=1913 |volume=14 |pages=1153–1156 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015021268936&view=1up&seq=1285&skin=2021 |trans-title=Measurements of the penetrating radiation in a free balloon at high altitudes |language=German}}</ref><ref>{{cite journal |last1=Kolhörster |first1=W. |title=Messungen der durchdringenden Strahlungen bis in Höhen von 9300 m. |journal=Verhandlungen der Deutschen Physikalischen Gesellschaft |date=1914 |volume=16 |pages=719–721 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924056112083&view=1up&seq=741&skin=2021 |trans-title=Measurements of the penetrating radiation up to heights of 9300 m. |language=German}}</ref>[[File:HessKol.jpg|thumb|Increase of ionization with altitude as measured by Hess in 1912 (left) and by Kolhörster (right)]] Hess received the [[Nobel Prize in Physics]] in 1936 for his discovery.<ref>{{cite web|author=Hess, V.F.|author-link=Victor Francis Hess|year=1936|title=The Nobel Prize in Physics 1936|publisher=[[The Nobel Foundation]]|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html|access-date=11 February 2010}}</ref><ref>{{cite web|author=Hess, V.F.|author-link=Victor Francis Hess|year=1936|title=Unsolved Problems in Physics: Tasks for the Immediate Future in Cosmic Ray Studies|series=Nobel Lectures|publisher=The Nobel Foundation|url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html|access-date=11 February 2010}}</ref>
[[File:Hessballon.jpg|thumb|Hess lands after his balloon flight in 1912.]]

===Identification===
[[Bruno Rossi]] wrote in 1964:
<blockquote>In the late 1920s and early 1930s the technique of self-recording electroscopes carried by balloons into the highest layers of the atmosphere or sunk to great depths under water was brought to an unprecedented degree of perfection by the German physicist [[Erich Regener]] and his group. To these scientists we owe some of the most accurate measurements ever made of cosmic-ray ionization as a function of altitude and depth.<ref>{{cite book|last=Rossi|first=Bruno Benedetto|title=Cosmic Rays|location=New York|publisher=McGraw-Hill|year=1964|isbn=978-0-07-053890-0}}</ref></blockquote>

[[Ernest Rutherford]] stated in 1931 that "thanks to the fine experiments of Professor Millikan and the even more far-reaching experiments of Professor Regener, we have now got for the first time, a curve of absorption of these radiations in water which we may safely rely upon".<ref>{{cite journal|last1=Geiger|first1=H.|last2=Rutherford|first2=Lord|last3=Regener|first3=E.|last4=Lindemann|first4=F.A.|last5=Wilson|first5=C.T.R.|last6=Chadwick|first6=J.|last7=Gray|first7=L.H.|last8=Tarrant|first8=G.T.P.|last9=Dobson|first9=G.M.B.|display-authors=6|year=1931|title=Discussion on Ultra-Penetrating Rays|journal=Proceedings of the Royal Society of London A|volume=132|issue=819|page=331|bibcode=1931RSPSA.132..331G|doi=10.1098/rspa.1931.0104|doi-access=free}}</ref>

In the 1920s, the term ''cosmic ray'' was coined by [[Robert Millikan]] who made measurements of ionization due to cosmic rays from deep under water to high altitudes and around the globe. Millikan believed that his measurements proved that the primary cosmic rays were gamma rays; i.e., energetic photons. And he proposed a theory that they were produced in interstellar space as by-products of the fusion of hydrogen atoms into the heavier elements, and that secondary [[electron]]s were produced in the atmosphere by [[Compton scattering]] of gamma rays. In 1927, while sailing from [[Java]] to the Netherlands, [[Jacob Clay]] found evidence,<ref>{{cite journal|author=Clay, J.|year=1927|title=Penetrating Radiation|journal=Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam| volume=30|issue=9–10|pages=1115–1127|url=http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-url=https://web.archive.org/web/20160206055736/http://www.dwc.knaw.nl/DL/publications/PU00011919.pdf |archive-date=2016-02-06 |url-status=live}}</ref> later confirmed in many experiments, that cosmic ray intensity increases from the tropics to mid-latitudes, which indicated that the primary cosmic rays are deflected by the geomagnetic field and must therefore be charged particles, not photons. In 1929, [[Walther Bothe|Bothe]] and [[Werner Kolhörster|Kolhörster]] discovered charged cosmic-ray particles that could penetrate 4.1&nbsp;cm of gold.<ref>{{cite journal|author1=Bothe, Walther| author2=Werner Kolhörster|date=November 1929|title=Das Wesen der Höhenstrahlung|journal=Zeitschrift für Physik|volume=56|issue=11–12|pages=751–777|doi=10.1007/BF01340137|bibcode=1929ZPhy...56..751B|s2cid=123901197}}</ref> Charged particles of such high energy could not possibly be produced by photons from Millikan's proposed interstellar fusion process.{{citation needed|date=April 2015}}

In 1930, [[Bruno Rossi]] predicted a difference between the intensities of cosmic rays arriving from the east and the west that depends upon the charge of the primary particles—the so-called "east–west effect".<ref>{{cite journal|title=On the Magnetic Deflection of Cosmic Rays|author=Rossi, Bruno|journal=Physical Review|date=August 1930|volume=36|issue=3|page=606|doi=10.1103/PhysRev.36.606|bibcode=1930PhRv...36..606R}}</ref> Three independent experiments<ref>{{cite journal|title=The Azimuthal Asymmetry of the Cosmic Radiation|author=Johnson, Thomas H.|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=834–835|doi=10.1103/PhysRev.43.834|bibcode=1933PhRv...43..834J}}</ref><ref>{{cite journal|title=A Positively Charged Component of Cosmic Rays|author=Alvarez, Luis|author2=Compton, Arthur Holly|journal=Physical Review|date=May 1933|volume=43|issue=10|pages=835–836|doi=10.1103/PhysRev.43.835|bibcode=1933PhRv...43..835A}}</ref><ref>{{cite journal|title=Directional Measurements on the Cosmic Rays Near the Geomagnetic Equator|author=Rossi, Bruno| journal=Physical Review|date=May 1934|volume=45|issue=3|pages=212–214|doi=10.1103/PhysRev.45.212|bibcode=1934PhRv...45..212R}}</ref> found that the intensity is, in fact, greater from the west, proving that most primaries are positive. During the years from 1930 to 1945, a wide variety of investigations confirmed that the primary cosmic rays are mostly protons, and the secondary radiation produced in the atmosphere is primarily electrons, photons and [[muon]]s. In 1948, observations with [[nuclear emulsion]]s carried by balloons to near the top of the atmosphere showed that approximately 10% of the primaries are helium nuclei (alpha particles) and 1% are nuclei of heavier elements such as carbon, iron, and lead.<ref>{{cite journal|title=Evidence for Heavy Nuclei in the Primary Cosmic radiation|author=Freier, Phyllis|journal=Physical Review|date=July 1948|volume=74|issue=2|pages=213–217|doi=10.1103/PhysRev.74.213|last2=Lofgren|first2=E.|last3=Ney|first3=E.|last4=Oppenheimer|first4=F.|last5=Bradt|first5=H.|last6=Peters|first6=B.|bibcode=1948PhRv...74..213F|display-authors=etal}}</ref><ref>{{cite journal|title=Investigation of the Primary Cosmic Radiation with Nuclear Photographic Emulsions|author=Freier, Phyllis|journal=Physical Review|date=December 1948|volume=74|issue=12|pages=1828–1837|doi=10.1103/PhysRev.74.1828|last2=Peters|first2=B.|bibcode=1948PhRv...74.1828B|display-authors=etal}}</ref>

During a test of his equipment for measuring the east–west effect, Rossi observed that the rate of near-simultaneous discharges of two widely separated [[Geiger counter]]s was larger than the expected accidental rate. In his report on the experiment, Rossi wrote "...&nbsp;it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another."<ref>{{cite journal|title=Misure sulla distribuzione angolare di intensita della radiazione penetrante all'Asmara|author=Rossi, Bruno|journal=Ricerca Scientifica|date=1934|volume=5|issue=1|pages=579–589}}</ref> In 1937, [[Pierre Victor Auger|Pierre Auger]], unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that high-energy primary cosmic-ray particles interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, and photons that reach ground level.<ref>{{citation|display-authors=1|last1=Auger|first1=P.|last2=Ehrenfest|first2=P.|last3=Maze|first3=R.|last4=Daudin|first4=J.|last5=Fréon|first5=R. A.|title=Extensive Cosmic-Ray Showers| journal=Reviews of Modern Physics|volume=11|issue=3–4|pages=288–291|date=July 1939|doi=10.1103/RevModPhys.11.288|bibcode=1939RvMP...11..288A| postscript=.}}</ref>

Soviet physicist [[Sergei Vernov]] was the first to use [[radiosonde]]s to perform cosmic ray readings with an instrument carried to high altitude by a balloon. On 1 April 1935, he took measurements at heights up to 13.6 kilometres using a pair of [[Geiger counter]]s in an anti-coincidence circuit to avoid counting secondary ray showers.<ref>{{cite book|author1=J.L. DuBois|author2=R.P. Multhauf|author3=C.A. Ziegler|date=2002|title=The Invention and Development of the Radiosonde|url=http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-url=https://web.archive.org/web/20110605132224/http://www.sil.si.edu/smithsoniancontributions/HistoryTechnology/pdf_lo/SSHT-0053.pdf |archive-date=2011-06-05 |url-status=live|series=Smithsonian Studies in History and Technology|volume=53|publisher=[[Smithsonian Institution Press]]}}</ref><ref>{{cite journal|author=S. Vernoff|date=1935|title=Radio-Transmission of Cosmic Ray Data from the Stratosphere|journal=Nature|volume=135|issue=3426|pages=1072–1073|doi=10.1038/1351072c0|bibcode=1935Natur.135.1072V|s2cid=4132258}}</ref>

[[Homi J. Bhabha]] derived an expression for the probability of scattering positrons by electrons, a process now known as [[Bhabha scattering]]. His classic paper, jointly with [[Walter Heitler]], published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs.<ref>{{cite journal|last1=Bhabha|first1=H. J.|last2=Heitler|first2=W.|title=The Passage of Fast Electrons and the Theory of Cosmic Showers|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=159|issue=898|year=1937|pages=432–458|issn=1364-5021|doi=10.1098/rspa.1937.0082|url=http://repository.ias.ac.in/2364/1/2364.pdf |archive-url=https://web.archive.org/web/20160102023556/http://repository.ias.ac.in/2364/1/2364.pdf |archive-date=2016-01-02 |url-status=live|bibcode=1937RSPSA.159..432B|doi-access=free}}</ref><ref>{{cite journal|last=Braunschweig|first=W.|display-authors=etal|title=A study of Bhabha scattering at PETRA energies|journal=Zeitschrift für Physik C|volume=37|issue=2|date=1988|pages=171–177|doi=10.1007/BF01579904|s2cid=121904361}}</ref>

===Energy distribution===
Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of ''density sampling'' and ''fast timing'' of [[air shower (physics)|extensive air showers]] were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the [[Massachusetts Institute of Technology]].<ref>{{Cite journal|last1=Clark|first1=G.|last2=Earl|first2=J.|last3=Kraushaar|first3=W.|last4=Linsley|first4=J.|last5=Rossi|first5=B.|last6=Scherb|first6=F.|last7=Scott|first7=D.|doi=10.1103/PhysRev.122.637|title=Cosmic-Ray Air Showers at Sea Level|journal=Physical Review|volume=122|issue=2|pages=637–654|year=1961|bibcode=1961PhRv..122..637C}}</ref> The experiment employed eleven [[Scintillator|scintillation detectors]] arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the [[Harvard College Observatory]]. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 10<sup>20</sup>&nbsp;eV. A huge air shower experiment called the [[Pierre Auger Observatory|Auger Project]] is currently operated at a site on the [[Pampa]]s of Argentina by an international consortium of physicists. The project was first led by [[James Cronin]], winner of the 1980 Nobel Prize in Physics from the [[University of Chicago]], and [[Alan Andrew Watson|Alan Watson]] of the [[University of Leeds]], and later by scientists of the international Pierre Auger Collaboration. Their aim is to explore the properties and arrival directions of the very highest-energy primary cosmic rays.<ref>{{cite web|url=https://www.auger.org/index.php/observatory|title=The Pierre Auger Observatory|publisher=Auger Project|url-status=live|archive-url=https://web.archive.org/web/20180903033644/https://www.auger.org/index.php/observatory|archive-date=3 September 2018}}</ref> The results are expected to have important implications for particle physics and cosmology, due to a theoretical [[Greisen–Zatsepin–Kuzmin limit]] to the energies of cosmic rays from long distances (about 160 million light years) which occurs above 10<sup>20</sup>&nbsp;eV because of interactions with the remnant photons from the [[Big Bang]] origin of the universe. Currently the Pierre Auger Observatory is undergoing an upgrade to improve its accuracy and find evidence for the yet unconfirmed origin of the most energetic cosmic rays.

High-energy gamma rays (>50{{nbsp}}MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967.<ref>{{cite journal|title=(none)|author=Kraushaar, W. L.| journal=The Astrophysical Journal|date=1972|volume=177|page=341|doi=10.1086/151713|bibcode=1972ApJ...177..341K|display-authors=etal|doi-access=free}}</ref> Components of both galactic and extra-galactic origins were separately identified at intensities much less than 1% of the primary charged particles. Since then, numerous satellite gamma-ray observatories have mapped the gamma-ray sky. The most recent is the Fermi Observatory, which has produced a map showing a narrow band of gamma ray intensity produced in discrete and diffuse sources in our galaxy, and numerous point-like extra-galactic sources distributed over the celestial sphere.

== Solar modulation==

Solar modulation theory explains how the intensity of cosmic rays changes as they travel through the heliosphere, influenced by the solar wind and magnetic field.<ref>{{cite journal |last1=Cummings |first1=A. C. |last2=Stone |first2=E. C. |title=Anomalous Cosmic Rays and Solar Modulation |journal=Space Science Reviews |date=1998 |volume=83 |issue=1–2 |pages=51–62 |doi=10.1023/A:1005057010311 |url=https://link.springer.com/article/10.1023/A:1005057010311|url-access=subscription }}</ref> The [[solar cycle]] causes variations in the magnetic field of the [[solar wind]] through which cosmic rays propagate to Earth. Solar modulation is a quasiperiodical change in cosmic rays intensity caused by 11- and 22-year cycles of solar activity.<ref>{{cite journal |last1=Tomassetti |first1=Nicola |last2=Bertucci |first2=Bruna |last3=Fiandrini |first3=Emanuele |title=Temporal evolution and rigidity dependence of the solar modulation lag of Galactic cosmic rays |journal=Physical Review D |date=2022 |volume=106 |issue=10 |article-number=103022 |doi=10.1103/PhysRevD.106.103022 |arxiv=2210.05693 |bibcode=2022PhRvD.106j3022T |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.106.103022}}</ref><ref>{{cite journal |last1=Potgieter |first1=Marius |title=Solar Modulation of Cosmic Rays |journal=Living Reviews in Solar Physics |date=2013 |volume=10 |issue=1 |page=3 |doi=10.12942/lrsp-2013-3 |arxiv=1306.4421 |bibcode=2013LRSP...10....3P |doi-access=free }}</ref>

== Parker transport equation ==
The Parker transport equation (also called the Parker equation, for [[Eugene Parker]]) is a kinetic equation that describes the acceleration and transport of energetic particles in astrophysical plasmas. The equation comprises diffusion terms both in coordinate space and in momentum space. The equation is used for studying energetic particle transport, and the mechanism of cosmic ray acceleration can be derived from it ([[Fermi acceleration|diffusive shock acceleration]]). The equation is also used "to study the acceleration of cosmic rays at supernova remnant shocks, the acceleration and transport of solar energetic particles at shocks driven by coronal mass ejections, and the acceleration and transport of energetic particles at the solar wind termination shock".<ref name=zimbardo>{{Cite journal|url=https://www.aanda.org/articles/aa/abs/2017/11/aa31179-17/aa31179-17.html|title=Fractional Parker equation for the transport of cosmic rays: steady-state solutions|first1=G.|last1=Zimbardo|first2=S.|last2=Perri|first3=F.|last3=Effenberger|first4=H.|last4=Fichtner|date=1 November 2017|journal=Astronomy & Astrophysics|volume=607|pages=A7|via=www.aanda.org|doi=10.1051/0004-6361/201731179 |bibcode=2017A&A...607A...7Z }}</ref>

The Parker transport equation in one dimension is:<ref name=zimbardo/>

:<math>\frac{\partial f}{\partial t} + U \frac{\partial f}{\partial x} - \frac{\partial}{\partial x}\left(k \frac{\partial f}{\partial x}\right) - \frac{p}{3}\frac{\partial U}{\partial x} \frac{\partial f}{\partial p} - \frac{1}{p^2}\frac{\partial}{\partial p}\left(p^2 D_{pp} \frac{\partial f}{\partial p}\right) = Q</math>

where:
* <math>f(x,p,t)</math> = omni-directional distribution function of energetic particles
* <math>U</math> = plasma (fluid) velocity
* <math>k(x,p)</math> = spatial diffusion coefficient
* <math>D_{pp}</math> = diffusion coefficient in momentum space
* <math>Q</math> = source term
* <math>p</math> = momentum
* <math>x</math> = position
* <math>t</math> = time

==Sources==
Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and [[Fritz Zwicky|Zwicky]] suggesting cosmic rays originated from supernovae.<ref>{{cite journal|last1=Baade|first1=W.|last2=Zwicky|first2=F.|year=1934|title=Cosmic rays from super-novae|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=20|issue=5|pages=259–263|jstor=86841|doi=10.1073/pnas.20.5.259|pmid=16587882|bibcode=1934PNAS...20..259B|pmc=1076396|doi-access=free}}</ref> A 1948 proposal by [[Horace W. Babcock]] suggested that magnetic variable stars could be a source of cosmic rays.<ref>{{cite journal|last1=Babcock|first1=H.|title=Magnetic variable stars as sources of cosmic rays|doi=10.1103/PhysRev.74.489|journal=Physical Review|volume=74|issue=4|page=489|year=1948|bibcode=1948PhRv...74..489B}}</ref> Subsequently, Sekido ''et al.'' (1951) identified the [[Crab Nebula]] as a source of cosmic rays.<ref>{{cite journal|last1=Sekido|first1=Y.|last2=Masuda|first2=T.|last3=Yoshida|first3=S.|last4=Wada|first4=M.|title=The Crab Nebula as an observed point source of cosmic rays|doi=10.1103/PhysRev.83.658.2|journal=Physical Review|volume=83|issue=3|pages=658–659|year=1951|bibcode=1951PhRv...83..658S}}</ref> Since then, a wide variety of potential sources for cosmic rays began to surface, including [[supernova]]e, active galactic nuclei, [[quasar]]s, and [[gamma-ray burst]]s.<ref>{{cite web|last=Gibb|first=Meredith|title=Cosmic rays|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|website=Imagine the Universe|publisher=NASA Goddard Space Flight Center|access-date=17 March 2013|date=3 February 2010}}</ref>

[[File:PIA16938-RadiationSources-InterplanetarySpace.jpg|thumb|left|Sources of ionizing radiation in interplanetary space.]]
Later experiments have helped to identify the sources of cosmic rays with greater certainty. In 2009, a paper presented at the [[International Cosmic Ray Conference]] by scientists at the Pierre Auger Observatory in Argentina showed ultra-high energy cosmic rays originating from a location in the sky very close to the [[radio galaxy]] [[Centaurus A]], although the authors specifically stated that further investigation would be required to confirm Centaurus&nbsp;A as a source of cosmic rays.<ref>{{cite conference|url=http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|title=Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects in Pierre Auger Observatory Data|access-date=17 March 2013|author=Hague, J.D.|book-title=Proceedings of the 31st ICRC, Łódź 2009|date=July 2009|conference=International Cosmic Ray Conference|location=Łódź, Poland|pages=6–9|archive-url=https://web.archive.org/web/20130528011624/http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|archive-date=28 May 2013}}</ref> However, no correlation was found between the incidence of gamma-ray bursts and cosmic rays, causing the authors to set upper limits as low as 3.4&nbsp;×&nbsp;10<sup>−6</sup>×&nbsp;[[erg]]·cm<sup>−2</sup> on the flux of {{nowrap|1 GeV – 1 TeV}} cosmic rays from gamma-ray bursts.<ref>{{cite journal|author=Hague, J.D.|title=Correlation of the highest energy cosmic rays with nearby extragalactic objects in Pierre Auger Observatory data|url=http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|journal=Proceedings of the 31st ICRC, Łódź, Poland 2009 – International Cosmic Ray Conference|date=July 2009|pages=36–39|access-date=17 March 2013|archive-url=https://web.archive.org/web/20130528011624/http://www.auger.org/technical_info/ICRC2009/arxiv_astrophysics.pdf|archive-date=28 May 2013}}</ref>

In 2009, supernovae were said to have been "pinned down" as a source of cosmic rays, a discovery made by a group using data from the [[Very Large Telescope]].<ref>{{cite web|url=http://www.space.com/6890-source-cosmic-rays-pinned.html|title=Source of cosmic rays pinned down|publisher=Tech Media Network|website=Space.com|date=25 June 2009|access-date=20 March 2013|author=Moskowitz, Clara|author-link= Clara Moskowitz}}</ref> This analysis, however, was disputed in 2011 with data from [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]], which revealed that "spectral shapes of [hydrogen and helium nuclei] are different and cannot be described well by a single power law", suggesting a more complex process of cosmic ray formation.<ref>{{cite journal|last1=Adriani|first1=O.|last2=Barbarino|first2=G.C.|last3=Bazilevskaya|first3=G.A.|last4=Bellotti|first4=R.|last5=Boezio|first5=M.|last6=Bogomolov|first6=E.A.|last7=Bonechi|first7=L.|last8=Bongi|first8=M.|last9=Bonvicini|first9=V.|last10=Borisov|doi=10.1126/science.1199172|first10=S.|last11=Bottai|first11=S.|last12=Bruno|first12=A.|last13=Cafagna|first13=F.|last14=Campana|first14=D.|last15=Carbone|first15=R.|last16=Carlson|first16=P.|last17=Casolino|first17=M.|last18=Castellini|first18=G.|last19=Consiglio|first19=L.|last20=de Pascale|first20=M.P.|last21=de Santis|first21=C.|last22=de Simone|first22=N.|last23=di Felice|first23=V.|last24=Galper|first24=A.M.|last25=Gillard|first25=W.|last26=Grishantseva|first26=L.|last27=Jerse|first27=G.|last28=Karelin|first28=A.V.|last29=Koldashov|first29=S.V.|last30=Krutkov|first30=S.Y.|title=PAMELA measurements of cosmic-ray proton and helium spectra|journal=Science|volume=332|issue=6025|pages=69–72|year=2011|pmid=21385721|arxiv=1103.4055|bibcode=2011Sci...332...69A|display-authors=6|hdl=2108/55474|s2cid=1234739}}</ref> In February 2013, though, research analyzing data from ''Fermi'' revealed through an observation of neutral pion decay that supernovae were indeed a source of cosmic rays, with each explosion producing roughly 3&nbsp;×&nbsp;10<sup>42</sup> – 3&nbsp;×&nbsp;10<sup>43</sup>{{nbsp}}[[joule|J]] of cosmic rays.<ref name=ackermann-2013/><ref name=pinholster-2013/>

[[File:Shockfrontacceleration.svg|thumb|Shock front acceleration (theoretical model for supernovae and active galactic nuclei): Incident proton gets accelerated between two shock fronts up to energies of the high-energy component of cosmic rays.]]
Supernovae do not produce all cosmic rays, however, and the proportion of cosmic rays that they do produce is a question which cannot be answered without deeper investigation.<ref>{{cite news|url=https://www.theguardian.com/science/2013/feb/14/cosmic-ray-mystery-solved|title=Cosmic ray mystery solved|publisher=Guardian News and Media Ltd|newspaper=[[The Guardian]]|place=London, UK|date=14 February 2013|access-date=21 March 2013|author=Jha, Alok}}</ref> To explain the actual process in supernovae and active galactic nuclei that accelerates the stripped atoms, physicists use shock front acceleration as a plausibility argument (see picture at right).

In 2017, the [[Pierre Auger Collaboration]] published the observation of a weak [[anisotropy]] in the arrival directions of the highest energy cosmic rays.<ref>{{cite journal|collaboration=The Pierre Auger Collaboration|title=Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8×10{{sup|18&nbsp;}}eV|journal=Science|year=2017|pages=1266–1270|volume=357|issue=6357|arxiv=1709.07321|doi=10.1126/science.aan4338|pmid=28935800|author1=Pierre Auger Collaboration|last2=Aab|first2=A.|last3=Abreu|first3=P.|last4=Aglietta|first4=M.|last5=Al Samarai|first5=I.|last6=Albuquerque|first6=I. F. M.|last7=Allekotte|first7=I.|last8=Almela|first8=A.|last9=Alvarez Castillo|first9=J.|last10=Alvarez-Muñiz|first10=J.|last11=Anastasi|first11=G. A.|last12=Anchordoqui|first12=L.|last13=Andrada|first13=B.|last14=Andringa|first14=S.|last15=Aramo|first15=C.|last16=Arqueros|first16=F.|last17=Arsene|first17=N.|last18=Asorey|first18=H.|last19=Assis|first19=P.|last20=Aublin|first20=J.|last21=Avila|first21=G.|last22=Badescu|first22=A. M.|last23=Balaceanu|first23=A.|last24=Barbato|first24=F.|last25=Barreira Luz|first25=R. J.|last26=Beatty|first26=J. J.|last27=Becker|first27=K. H.|last28=Bellido|first28=J. A.|last29=Berat|first29=C.|last30=Bertaina|first30=M. E.|bibcode=2017Sci...357.1266P|s2cid=3679232|display-authors=29}}</ref> Since the Galactic Center is in the deficit region, this anisotropy can be interpreted as evidence for the extragalactic origin of cosmic rays at the highest energies. This implies that there must be a transition energy from galactic to extragalactic sources, and there may be different types of cosmic-ray sources contributing to different energy ranges.

==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}}</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|article-number=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|article-number=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|article-number=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 |hdl-access=free}}</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}}</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|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}}</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>

==Detection methods==
[[File:VERITAS array.jpg|thumb|600px|The [[VERITAS]] array of air Cherenkov telescopes.]]
There are two main classes of detection methods. First, the direct detection of the primary cosmic rays in space or at high altitude by balloon-borne instruments. Second, the indirect detection of secondary particle, i.e., extensive air showers at higher energies. While there have been proposals and prototypes for space and balloon-borne detection of air showers, currently operating experiments for high-energy cosmic rays are ground based. Generally direct detection is more accurate than indirect detection. However the flux of cosmic rays decreases with energy, which hampers direct detection for the energy range above 1 PeV. Both direct and indirect detection are realized by several techniques.

===Direct detection===
Direct detection is possible by all kinds of particle detectors at the [[ISS]], on satellites, or high-altitude balloons. However, there are constraints in weight and size limiting the choices of detectors.

An example for the direct detection technique is a method based on [[nuclear track]]s developed by Robert Fleischer, [[P. Buford Price|P.&nbsp;Buford Price]], and [[Robert M. Walker (physicist)|Robert&nbsp;M. Walker]] for use in high-altitude balloons.<ref>{{cite book|author1=R.L. Fleischer|author2=P.B. Price|author3=R.M. Walker|date=1975|title=Nuclear tracks in solids: Principles and applications|publisher=[[University of California Press]]|bibcode=1975ucb..book.....F }}</ref> In this method, sheets of clear plastic, like 0.25&nbsp;[[millimetre|mm]] [[Lexan]] polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or [[ionization]] in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path. The resulting plastic sheets are "etched" or slowly dissolved in warm caustic [[sodium hydroxide]] solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves the plastic at a faster rate along the path of the ionized plastic. The net result is a conical etch pit in the plastic. The etch pits are measured under a high-power microscope (typically 1600× oil-immersion), and the etch rate is plotted as a function of the depth in the stacked plastic.

This technique yields a unique curve for each atomic nucleus from 1 to&nbsp;92, allowing identification of both the charge and energy of the cosmic ray that traverses the plastic stack. The more extensive the ionization along the path, the higher the charge. In addition to its uses for cosmic-ray detection, the technique is also used to detect nuclei created as products of [[nuclear fission]].

===Indirect detection===
There are several ground-based methods of detecting cosmic rays currently in use, which can be divided in two main categories: the detection of secondary particles forming extensive air showers (EAS) by various types of particle detectors, and the detection of electromagnetic radiation emitted by EAS in the atmosphere.

Extensive air shower arrays made of particle detectors measure the charged particles which pass through them. EAS arrays can observe a broad area of the sky and can be active more than 90% of the time. However, they are less able to segregate background effects from cosmic rays than can [[IACT|air Cherenkov telescopes]]. Most state-of-the-art EAS arrays employ plastic [[scintillator]]s. Also water (liquid or frozen) is used as a detection medium through which particles pass and produce [[Cherenkov radiation]] to make them detectable.<ref>{{cite web|url=http://www.nscl.msu.edu/files/PAN%20cosmic%20ray%20articles.pdf|title=What are cosmic rays?|publisher=Michigan State University National Superconducting Cyclotron Laboratory|access-date=23 February 2013|archive-url=https://web.archive.org/web/20120712052716/http://www.nscl.msu.edu/files/PAN%20cosmic%20ray%20articles.pdf|archive-date=12 July 2012}}</ref> Therefore, several arrays use water/ice-Cherenkov detectors as alternative or in addition to scintillators.
By the combination of several detectors, some EAS arrays have the capability to distinguish muons from lighter secondary particles (photons, electrons, positrons). The fraction of muons among the secondary particles is one traditional way to estimate the mass composition of the primary cosmic rays.

An historic method of secondary particle detection still used for demonstration purposes involves the use of [[cloud chamber]]s<ref>{{cite web|url=https://www.chess.cornell.edu/Outreach/document/cloudchamber.pdf |archive-url=https://web.archive.org/web/20130606193736/http://www.chess.cornell.edu/Outreach/document/cloudchamber.pdf |archive-date=2013-06-06 |url-status=live|title=Cloud Chambers and Cosmic Rays: A Lesson Plan and Laboratory Activity for the High School Science Classroom|publisher=[[Cornell University]] [[Laboratory for Elementary-Particle Physics]]|date=2006|access-date=23 February 2013}}</ref> to detect the secondary muons created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory. A fifth method, involving [[bubble chamber]]s, can be used to detect cosmic ray particles.<ref>{{Cite journal|last1=Chu|first1=W.|last2=Kim|first2=Y.|last3=Beam|first3=W.|last4=Kwak|first4=N.|title=Evidence of a Quark in a High-Energy Cosmic-Ray Bubble-Chamber Picture|doi=10.1103/PhysRevLett.24.917|journal=Physical Review Letters|volume=24|issue=16|pages=917–923|year=1970|bibcode=1970PhRvL..24..917C}}</ref>

More recently, the [[CMOS]] devices in pervasive [[smartphone]] cameras have been proposed as a practical distributed network to detect air showers from ultra-high-energy cosmic rays.<ref>{{cite web|url=https://arstechnica.com/science/2014/10/cosmic-ray-particle-shower-theres-an-app-for-that/|title=Cosmic ray particle shower? There's an app for that.|work=Ars Technica|last=Timmer|first=John|date=13 October 2014}}</ref> The first [[mobile app|app]] to exploit this proposition was the CRAYFIS (Cosmic RAYs Found in Smartphones) experiment.<ref>[http://crayfis.ps.uci.edu/ Collaboration website] {{webarchive|url=https://web.archive.org/web/20141014162644/http://crayfis.ps.uci.edu/|date=14 October 2014}}</ref><ref>[http://crayfis.ps.uci.edu/paper.pdf CRAYFIS detector array paper. ] {{webarchive|url=https://web.archive.org/web/20141014162639/http://crayfis.ps.uci.edu/paper.pdf|date=14 October 2014}}</ref> In 2017, the CREDO ([[Cosmic-Ray Extremely Distributed Observatory]]) Collaboration<ref>{{Cite web|url=https://credo.science/|title=CREDO|website=credo.science}}</ref> released the first version of its completely open source app for Android devices. Since then the collaboration has attracted the interest and support of many scientific institutions, educational institutions, and members of the public around the world.<ref>{{Cite web|url=https://www.eurekalert.org/pub_releases/2018-10/thni-cfl100418.php|title=CREDO's first light: The global particle detector begins its collection of scientific data|website=EurekAlert!}}</ref> Future research has to show in what aspects this new technique can compete with dedicated EAS arrays.

The first detection method in the second category is called the [[IACT|air Cherenkov telescope]], designed to detect low-energy (<200&nbsp;GeV) cosmic rays by means of analyzing their [[Cherenkov radiation]], which for cosmic rays are gamma rays emitted as they travel faster than the [[speed of light]] in their medium, the atmosphere.<ref>{{cite web|url=http://www.lanl.gov/milagro/detecting.shtml|title=The Detection of Cosmic Rays|publisher=Los Alamos National Laboratory|work=Milagro Gamma-Ray Observatory|access-date=22 February 2013|date=3 April 2002|archive-url=https://web.archive.org/web/20130305045916/http://www.lanl.gov/milagro/detecting.shtml|archive-date=5 March 2013}}</ref> While these telescopes are extremely good at distinguishing between background radiation and that of cosmic-ray origin, they can only function well on clear nights without the Moon shining, have very small fields of view, and are only active for a few percent of the time.

A second method detects the light from nitrogen fluorescence caused by the excitation of nitrogen in the atmosphere by particles moving through the atmosphere. This method is the most accurate for cosmic rays at highest energies, in particular when combined with EAS arrays of particle detectors.<ref>{{cite journal|last1=Letessier-Selvon|first1=Antoine|last2=Stanev|first2=Todor|title=Ultrahigh energy cosmic rays|journal=Reviews of Modern Physics|volume=83|issue=3|pages=907–942|doi=10.1103/RevModPhys.83.907|bibcode=2011RvMP...83..907L|arxiv=1103.0031|year=2011|s2cid=119237295}}</ref> Similar to the detection of Cherenkov-light, this method is restricted to clear nights.

Another method detects radio waves emitted by air showers. This technique has a high duty cycle similar to that of particle detectors. The accuracy of this technique was improved in the last years as shown by various prototype experiments, and may become an alternative to the detection of atmospheric Cherenkov-light and fluorescence light, at least at high energies.

==Effects==

===Changes in atmospheric chemistry===
Cosmic rays ionize nitrogen and oxygen molecules in the atmosphere, which leads to a number of chemical reactions. Cosmic rays are also responsible for the continuous production of a number of [[Radionuclide|unstable isotopes]], such as [[carbon-14]], in the Earth's atmosphere through the reaction:

{{block indent|n + <sup>14</sup>N → p + <sup>14</sup>C}}

Cosmic rays kept the level of carbon-14<ref>{{cite book|last=Trumbore|first=Susan|author-link=Susan Trumbore|title=Quaternary Geochronology: Methods and Applications|year=2000|publisher=American Geophysical Union|location=Washington, D.C.|isbn=978-0-87590-950-9|pages=41–59|url=http://www.agu.org/books/rf/v004/|editor=J. S. Noller|editor2=J. M. Sowers|editor3=W. R. Lettis|access-date=28 October 2011|archive-date=21 May 2013|archive-url=https://web.archive.org/web/20130521094616/http://www.agu.org/books/rf/v004/}}</ref> in the atmosphere roughly constant (70 tons) for at least the past 100,000 years,{{citation needed|date=July 2017}} until the beginning of above-ground nuclear weapons testing in the early 1950s. This fact is used in [[radiocarbon dating]].

====Reaction products of primary cosmic rays, radioisotope half-lifetime, and production reaction====
<!-- Converting this to a table would be nice-->
{{columns-list|colwidth=30em|
* [[Hydrogen-1]] (stable): [[cosmic ray spallation|spallation]] from nitrogen and oxygen, decay of neutrons from such spallation
* [[Helium-3]] (stable): spallation or from tritium
* [[Helium-4]] (stable): spallation producing alpha rays
* [[Tritium]] (12.3 years): <sup>14</sup>N(n, <sup>3</sup>H)<sup>12</sup>C (spallation)
* Beryllium-7 (53.3 days)
* [[Beryllium-10]] (1.39 million years): <sup>14</sup>N(n,p α)<sup>10</sup>Be (spallation)
* [[Carbon-14]] (5730 years): <sup>14</sup>N(n, p)<sup>14</sup>C ([[neutron activation]])
* Sodium-22 (2.6 years)
* Sodium-24 (15 hours)
* Magnesium-28 (20.9 hours)
* Silicon-31 (2.6 hours)
* Silicon-32 (101 years)
* [[Phosphorus-32]] (14.3 days)
* Sulfur-35 (87.5 days)
* Sulfur-38 (2.84 hours)
* Chlorine-34 m (32 minutes)
* [[Chlorine-36]] (300,000 years)
* Chlorine-38 (37.2 minutes)
* Chlorine-39 (56 minutes)
* Argon-39 (269 years)
* [[Krypton-85]] (10.7 years)<ref>{{cite web|title=Natürliche, durch kosmische Strahlung laufend erzeugte Radionuklide|url=http://www.um.baden-wuerttemberg.de/servlet/is/34839/Natuerliche_durch_kosmische_Strahlung_laufend_erzeugte_Radionuklide.pdf?command=downloadContent&filename=Natuerliche_durch_kosmische_Strahlung_laufend_erzeugte_Radionuklide.pdf|access-date=11 February 2010|language=de|archive-url=https://web.archive.org/web/20100203174117/http://www.um.baden-wuerttemberg.de/servlet/is/34839/Natuerliche_durch_kosmische_Strahlung_laufend_erzeugte_Radionuklide.pdf?command=downloadContent&filename=Natuerliche_durch_kosmische_Strahlung_laufend_erzeugte_Radionuklide.pdf|archive-date=3 February 2010}}</ref>
}}

===Role in ambient radiation===
Cosmic rays constitute a fraction of the annual radiation exposure of human beings on the Earth, averaging 0.39{{nbsp}}mSv out of a total of 3{{nbsp}}mSv per year (13% of total background) for the Earth's population. However, the background radiation from cosmic rays increases with altitude, from 0.3{{nbsp}}mSv per year for sea-level areas to 1.0{{nbsp}}mSv per year for higher-altitude cities, raising cosmic radiation exposure to a quarter of total background radiation exposure for populations of said cities. Airline crews flying long-distance high-altitude routes can be exposed to 2.2{{nbsp}}mSv of extra radiation each year due to cosmic rays, nearly doubling their total exposure to ionizing radiation.

{| class="wikitable" style="text-align:center"
|- style="background:#ececec;"
|+ Average annual radiation exposure ([[millisievert]]s)
|-
! colspan="2" |Radiation
! colspan="2" |[[UNSCEAR]]<ref>UNSCEAR [http://www.unscear.org/docs/reports/2008/09-86753_Report_2008_Annex_B.pdf "Sources and Effects of Ionizing Radiation"] page 339 retrieved 29 June 2011</ref><ref>[[:ja:放射線医学総合研究所|Japan NIRS]] [http://www.aec.go.jp/jicst/NC/iinkai/teirei/siryo2010/siryo59/siryo1.pdf UNSCEAR 2008 report] page 8 retrieved 29 June 2011</ref>
! Princeton<ref>Princeton.edu [http://web.princeton.edu/sites/ehs/osradtraining/backgroundradiation/background.htm "Background radiation"] {{Webarchive|url=https://web.archive.org/web/20110609095603/http://web.princeton.edu/sites/ehs/osradtraining/backgroundradiation/background.htm|date=9 June 2011}} retrieved 29 June 2011</ref>
! Wa State<ref>Washington state Dept. of Health [http://www.doh.wa.gov/ehp/rp/factsheets/factsheets-htm/fs10bkvsman.htm "Background radiation"] {{webarchive|url=https://web.archive.org/web/20120502102254/http://www.doh.wa.gov/ehp/rp/factsheets/factsheets-htm/fs10bkvsman.htm |date=2 May 2012 }} retrieved 29 June 2011</ref>
! MEXT<ref>Ministry of Education, Culture, Sports, Science, and Technology of Japan [http://www.kankyo-hoshano.go.jp/04/04-1.html "Radiation in environment"] {{Webarchive|url=https://web.archive.org/web/20110322231148/http://www.kankyo-hoshano.go.jp/04/04-1.html |date=22 March 2011 }} retrieved 29 June 2011</ref>
! rowspan="2" | Remark
|-
! Type !! Source !! World<br /> average !! Typical range !! US !! US !! Japan
|-
| rowspan="5" | Natural || Air || 1.26 || 0.2–10.0<sup>a</sup> || 2.29 || 2.00 || 0.40 || <small>Primarily from radon,</small> <sup>(a)</sup><small>depends on indoor accumulation of radon gas.</small>
|-
| Internal || 0.29 || 0.2–1.0<sup>b</sup> || 0.16 || 0.40 || 0.40 || <small>Mainly from radioisotopes in food ([[potassium-40|<sup>40</sup>K]], <sup>14</sup>C, etc.)</small> <sup>(b)</sup><small>depends on diet.</small>
|-
| Terrestrial || 0.48 || 0.3–1.0<sup>c</sup> || 0.19 || 0.29 || 0.40 || <sup>(c)</sup><small>Depends on soil composition and building material of structures.</small>
|- style="background:orange;"
| Cosmic || 0.39 || 0.3–1.0<sup>d</sup> || 0.31 || 0.26 || 0.30 || <sup>(d)</sup><small>Generally increases with elevation.</small>
|-
| '''Subtotal''' || 2.40 || 1.0–13.0 || 2.95 || 2.95 || 1.50 ||
|-
| rowspan="4" | Artificial || Medical || 0.60 || 0.03–2.0 || 3.00 || 0.53 || 2.30 ||
|-
| Fallout || 0.007 || 0–1+ || – || – || 0.01 || <small>Peaked in 1963 (prior to the [[Partial Test Ban Treaty]]) with [[Chernobyl nuclear accident|a spike in 1986]]; still high near nuclear test and accident sites.<br />For the United States, fallout is incorporated into other categories.</small>
|-
| Others || 0.0052 || 0–20 || 0.25 || 0.13 || 0.001|| <small>Average annual occupational exposure is 0.7 mSv; mining workers have higher exposure. <br />Populations near nuclear plants have an additional ≈0.02 mSv of exposure annually.</small>
|-
| '''Subtotal''' || 0.6 || 0 to tens || 3.25 || 0.66 || 2.311 ||
|-
| colspan="2" | '''Total''' || 3.00 || 0 to tens || 6.20 || 3.61 || 3.81 ||
|}
<small>Figures are for the time before the [[Fukushima Daiichi nuclear disaster]]. Human-made values by UNSCEAR are from the Japanese National Institute of Radiological Sciences, which summarized the UNSCEAR data.</small>

===Effect on electronics===
{{See also|Radiation hardening}}Cosmic rays have sufficient energy to alter the states of circuit components in [[electronics|electronic]] [[integrated circuit]]s, causing transient errors to occur (such as corrupted data in [[random-access memory|electronic memory devices]] or incorrect performance of [[CPU]]s) often referred to as "[[soft error]]s". This has been a problem in [[electronics]] at extremely high-altitude, such as in [[satellite]]s, but with [[transistor]]s becoming smaller and smaller, this is becoming an increasing concern in ground-level electronics as well.<ref>[http://www.research.ibm.com/journal/rd/401/curtis.html "IBM experiments in soft fails in computer electronics (1978–1994)"]. In [http://www.research.ibm.com/journal/rd40-1.html "Terrestrial cosmic rays and soft errors"], ''IBM Journal of Research and Development'', Vol. 40, No. 1, 1996. Retrieved 16 April 2008.</ref> Studies by [[IBM]] in the 1990s suggest that computers typically experience about one cosmic-ray-induced error per 256 megabytes of [[Random-access memory|RAM]] per month.<ref>{{cite web|author=Scientific American|author-link=Scientific American|date=21 July 2008|title=Solar Storms: Fast Facts|url=https://www.scientificamerican.com/article/solar-storms-fast-facts/|publisher=[[Nature Publishing Group]]}}</ref> To alleviate this problem, the [[Intel Corporation]] has proposed a cosmic ray detector that could be integrated into future high-density [[microprocessor]]s, allowing the processor to repeat the last command following a cosmic-ray event.<ref>[https://news.bbc.co.uk/2/hi/technology/7335322.stm "Intel plans to tackle cosmic ray threat"]. ''BBC News'', 8 April 2008. Retrieved 16 April 2008.</ref> [[ECC memory]] is used to protect data against data corruption caused by cosmic rays.

In 2008, data corruption in a flight control system caused an [[Airbus A330]] airliner to twice [[Qantas Flight 72|plunge hundreds of feet]], resulting in injuries to multiple passengers and crew members. Cosmic rays were investigated among other possible causes of the data corruption, but were ultimately ruled out as being very unlikely.<ref>[https://www.atsb.gov.au/media/3532398/ao2008070.pdf "In-flight upset, 154&nbsp;km west of Learmonth, Western Australia, 7 October 2008, VH-QPA, Airbus A330-303"] {{Webarchive|url=https://web.archive.org/web/20220505014942/https://www.atsb.gov.au/media/3532398/ao2008070.pdf |date=5 May 2022 }} (2011). Australian Transport Safety Bureau.</ref>

In August 2020, scientists reported that ionizing radiation from environmental radioactive materials and cosmic rays may substantially limit the [[Quantum decoherence|coherence]] times of [[qubit]]s if they are not shielded adequately which may be critical for realizing fault-tolerant superconducting [[quantum computer]]s in the future.<ref>{{cite news|title=Quantum computers may be destroyed by high-energy particles from space|url=https://www.newscientist.com/article/2252933-quantum-computers-may-be-destroyed-by-high-energy-particles-from-space/|access-date=7 September 2020|work=New Scientist}}</ref><ref>{{cite news|title=Cosmic rays may soon stymie quantum computing|url=https://phys.org/news/2020-08-cosmic-rays-stymie-quantum.html|access-date=7 September 2020|work=phys.org}}</ref><ref>{{cite journal|last1=Vepsäläinen|first1=Antti P.|last2=Karamlou|first2=Amir H.|last3=Orrell|first3=John L.|last4=Dogra|first4=Akshunna S.|last5=Loer|first5=Ben|last6=Vasconcelos|first6=Francisca|last7=Kim |first7=David K.|last8=Melville|first8=Alexander J.|last9=Niedzielski|first9=Bethany M.|last10=Yoder|first10=Jonilyn L.|last11=Gustavsson|first11=Simon|last12=Formaggio|first12=Joseph A.|last13=VanDevender|first13=Brent A.|last14=Oliver|first14=William D.|title=Impact of ionizing radiation on superconducting qubit coherence|journal=Nature|date=August 2020|volume=584|issue=7822 |pages=551–556|doi=10.1038/s41586-020-2619-8|pmid=32848227|arxiv=2001.09190|bibcode=2020Natur.584..551V|s2cid=210920566|url=https://www.nature.com/articles/s41586-020-2619-8|access-date=7 September 2020|issn=1476-4687}}</ref>

===Significance to aerospace travel===
{{Main|Health threat from cosmic rays}}
Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft. Cosmic rays also pose a threat to electronics placed aboard outgoing probes. In 2010, a malfunction aboard the ''[[Voyager 2]]'' space probe was credited to a single [[Single-event upset|flipped bit]], probably caused by a cosmic ray. Strategies such as physical or magnetic shielding for spacecraft have been considered in order to minimize the damage to electronics and human beings caused by cosmic rays.<ref>{{cite web|url=http://settlement.arc.nasa.gov/75SummerStudy/5appendE.html|archive-url=https://web.archive.org/web/20100531210412/http://settlement.arc.nasa.gov/75SummerStudy/5appendE.html|archive-date=31 May 2010|title=Appendix E: Mass Shielding|publisher=NASA|work=Space Settlements: A Design Study|date=10 July 2002|access-date=24 February 2013|author=Globus, Al}}</ref><ref>{{cite news|url=http://www.thespacereview.com/article/308/1|title=Magnetic shielding for spacecraft|work=The Space Review|date=24 January 2005|access-date=24 February 2013|author=Atkinson, Nancy}}</ref>

On 31 May 2013, NASA scientists reported that a possible [[Human mission to Mars|crewed mission to Mars]] may involve a greater radiation risk than previously believed, based on the amount of [[radiation|energetic particle radiation]] detected by the [[Radiation assessment detector|RAD]] on the [[Mars Science Laboratory]] while traveling from the [[Earth]] to [[Mars]] in 2011–2012.<ref name=SCI-20130531a/><ref name=SCI-20130531b/><ref name=NYT-20130530/>

[[File:PIA17601-Comparisons-RadiationExposure-MarsTrip-20131209.png|thumb|upright=1.1|left|Comparison of radiation doses, including the amount detected on the trip from Earth to Mars by the [[radiation assessment detector|RAD]] on the [[Mars Science Laboratory|MSL]] (2011–2013).<ref name="SCI-20130531a">{{cite journal|last=Kerr|first=Richard|title=Radiation Will Make Astronauts' Trip to Mars Even Riskier|date=31 May 2013|journal=Science|volume=340|page=1031|doi=10.1126/science.340.6136.1031|issue=6136|pmid=23723213|bibcode=2013Sci...340.1031K}}</ref><ref name="SCI-20130531b">{{cite journal|title=Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory|journal=Science|date=31 May 2013|volume=340|pages=1080–1084|doi=10.1126/science.1235989|pmid=23723233|author=Zeitlin, C.|issue=6136|last2=Hassler|first2=D. M.|last3=Cucinotta |first3=F. A.|last4=Ehresmann|first4=B.|last5=Wimmer-Schweingruber|first5=R.F.|last6=Brinza|first6=D. E.|last7=Kang|first7=S.|last8=Weigle|first8=G.|last9=Bottcher|first9=S.|display-authors=8|bibcode=2013Sci...340.1080Z|s2cid=604569}}</ref><ref name="NYT-20130530">{{cite news|last=Chang|first=Kenneth|title=Data Point to Radiation Risk for Travelers to Mars|url=https://www.nytimes.com/2013/05/31/science/space/data-show-higher-cancer-risk-for-mars-astronauts.html|date=30 May 2013|work=[[The New York Times]]|access-date=31 May 2013}}</ref>]]

Flying {{convert|12|km|ft|}} high, passengers and crews of [[jet airliner]]s are exposed to at least 10 times the cosmic ray dose that people at [[sea level]] receive. Aircraft flying [[polar route]]s near the [[geomagnetic pole]]s are at particular risk.<ref>{{cite web|last=Phillips|first=Tony|title=The Effects of Space Weather on Aviation|work=Science News|publisher=NASA|date=25 October 2013|url=https://science.nasa.gov/science-news/science-at-nasa/2013/25oct_aviationswx/|access-date=12 July 2017|archive-date=28 September 2019|archive-url=https://web.archive.org/web/20190928003535/https://science.nasa.gov/science-news/science-at-nasa/2013/25oct_aviationswx/}}</ref><ref>{{YouTube|bAKdaYumlq4|"Converting Cosmic Rays to Sound During a Transatlantic Flight to Zurich"}}</ref><ref>{{Cite web|url=http://sol.spacenvironment.net/raps_ops/current_files/globeView.html|title=NAIRAS Real-time radiation Dose|website=sol.spacenvironment.net}}</ref>

===Role in lightning===
Cosmic rays have been implicated in the triggering of electrical breakdown in [[lightning]]. It has been proposed that essentially all lightning is triggered through a relativistic process, or "[[runaway breakdown]]", seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.<ref>[http://physicstoday.scitation.org/doi/pdf/10.1063/1.1995746 "Runaway Breakdown and the Mysteries of Lightning"], ''Physics Today'', May 2005.</ref>

===Postulated role in climate change===
A role for cosmic rays in climate was suggested by [[Edward P. Ney]] in 1959<ref>{{cite journal| title=Cosmic Radiation and the Weather|journal=Nature|date=14 February 1959|first=Edward P.|last=Ney|volume=183|pages=451–452|bibcode=1959Natur.183..451N|doi=10.1038/183451a0|issue=4659|s2cid=4157226}}</ref> and by [[Robert E. Dickinson]] in 1975.<ref>{{cite journal|title=Solar Variability and the Lower Atmosphere|last=Dickinson |first=Robert E.|journal=Bulletin of the American Meteorological Society|date=December 1975|volume=56|issue=12|pages=1240–1248|doi=10.1175/1520-0477(1975)056<1240:SVATLA>2.0.CO;2|bibcode=1975BAMS...56.1240D|doi-access=free}}</ref> It has been postulated that cosmic rays may have been responsible for major climatic change and mass extinction in the past. According to Adrian Mellott and Mikhail Medvedev, 62-million-year cycles in biological marine populations correlate with the motion of the Earth relative to the galactic plane and increases in exposure to cosmic rays.<ref>{{Cite web|url=http://news.nationalgeographic.com/news/2007/04/070420-extinctions.html|archive-url=https://web.archive.org/web/20070423085401/http://news.nationalgeographic.com/news/2007/04/070420-extinctions.html|archive-date=23 April 2007|title=Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say |work=National Geographic |year=2007}}</ref> The researchers suggest that this and gamma ray bombardments deriving from local supernovae could have affected [[cancer]] and [[mutation rate]]s, and might be linked to decisive alterations in the Earth's climate, and to the [[Extinction event|mass extinctions]] of the [[Ordovician]].<ref>{{cite journal|last1=Melott |first1=A. L.|last2=Thomas |first2=B. C.|year=2009|title=Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage|journal=Paleobiology|volume=35|pages=311–320|doi=10.1666/0094-8373-35.3.311|issue=3|arxiv=0809.0899|bibcode=2009Pbio...35..311M |s2cid=11942132}}</ref><ref>{{Cite web|url=https://www.space.com/33379-supernova-explosions-earth-life-mass-extinction.html|title=Did Supernova Explosion Contribute to Earth Mass Extinction?|website=Space.com|date=11 July 2016}}</ref>

Danish physicist [[Henrik Svensmark]] has controversially argued that because [[solar variation]] modulates the cosmic ray flux on Earth, it would consequently affect the rate of cloud formation and hence be an indirect cause of [[global warming]].<ref>{{cite web|last=Long|first=Marion|title=Sun's Shifts May Cause Global Warming|publisher=[[Discover (magazine)|Discover]]|date=25 June 2007|url=http://discovermagazine.com/2007/jul/the-discover-interview-henrik-svensmark/|access-date=7 July 2013}}</ref><ref>{{Cite journal| first=Henrik |last=Svensmark|title=Influence of Cosmic Rays on Earth's Climate|journal=[[Physical Review Letters]]|year=1998|volume=81|pages=5027–5030|doi=10.1103/PhysRevLett.81.5027|issue=22|bibcode=1998PhRvL..81.5027S|url=http://ruby.fgcu.edu/courses/twimberley/EnviroPol/EnviroPhilo/Svensmark.pdf |archive-url=https://web.archive.org/web/20170809142617/http://ruby.fgcu.edu/courses/twimberley/EnviroPol/EnviroPhilo/Svensmark.pdf |archive-date=2017-08-09 |url-status=live|citeseerx=10.1.1.522.585}}</ref> Svensmark is one of several scientists outspokenly opposed to the mainstream scientific assessment of global warming, leading to concerns that the proposition that cosmic rays are connected to global warming could be ideologically biased rather than scientifically based.<ref>{{cite news|last1=Plait|first1=Phil|title=No, a new study does not show cosmic-rays are connected to global warming|url=http://blogs.discovermagazine.com/badastronomy/2011/08/31/no-a-new-study-does-not-show-cosmic-rays-are-connected-to-global-warming/|access-date=11 January 2018|work=Discover|publisher=Kalmbach|date=31 August 2011|archive-date=12 January 2018|archive-url=https://web.archive.org/web/20180112042455/http://blogs.discovermagazine.com/badastronomy/2011/08/31/no-a-new-study-does-not-show-cosmic-rays-are-connected-to-global-warming/}}</ref> Other scientists have vigorously criticized Svensmark for sloppy and inconsistent work: one example is adjustment of cloud data that understates error in lower cloud data, but not in high cloud data;<ref>{{Cite web|last=Benestad|first=Rasmus E.|title='Cosmoclimatology' – tired old arguments in new clothes|date=9 March 2007|url=http://www.realclimate.org/index.php/archives/2007/03/cosmoclimatology-tired-old-arguments-in-new-clothes/|access-date=13 November 2013}}</ref> another example is "incorrect handling of the physical data" resulting in graphs that do not show the correlations they claim to show.<ref>Peter Laut, [http://stephenschneider.stanford.edu/Publications/PDF_Papers/Laut2003.pdf "Solar activity and terrestrial climate: an analysis of some purported correlations"], ''Journal of Atmospheric and Solar-Terrestrial Physics'' 65 (2003) 801–812</ref> Despite Svensmark's assertions, galactic cosmic rays have shown no statistically significant influence on changes in cloud cover,<ref>{{cite journal|last1=Lockwood|first1=Mike|title=Solar Influence on Global and Regional Climates|journal=Surveys in Geophysics|date=16 May 2012|volume=33|issue=3–4|pages=503–534|doi=10.1007/s10712-012-9181-3|bibcode=2012SGeo...33..503L|doi-access=free}}</ref> and have been demonstrated in studies to have no causal relationship to changes in global temperature.<ref>{{cite journal|last1=Sloan|first1=T.|last2=Wolfendale|first2=A. W.|author-link2=Arnold Wolfendale|title=Cosmic rays, solar activity and the climate|journal=Environmental Research Letters|date=7 November 2013|volume=8|issue=4|article-number=045022|doi=10.1088/1748-9326/8/4/045022|bibcode=2013ERL.....8d5022S|doi-access=free}}</ref>

===Possible mass extinction factor===
{{See also|Pliocene#Supernovae}}
A handful of studies conclude that a nearby supernova or series of supernovas caused the [[Pliocene]] marine megafauna extinction event by substantially increasing radiation levels to hazardous amounts for large seafaring animals.<ref>{{cite journal|last1=Melott|first1=Adrian L.|first2=F. |last2=Marinho|first3=L. |last3=Paulucci|title=Muon Radiation Dose and Marine Megafaunal Extinction at the end-Pliocene Supernova|journal=Astrobiology|volume=19|issue=6|pages=825–830|date=2019|doi=10.1089/ast.2018.1902|pmid=30481053|arxiv=1712.09367|s2cid=33930965}}</ref><ref>{{cite journal|last=Benitez|first=Narciso|date=2002|title=Evidence for Nearby Supernova Explosions|journal=Physical Review Letters|volume=88|issue=8|article-number=081101|doi=10.1103/PhysRevLett.88.081101|pmid=11863949|display-authors=et al.|arxiv=astro-ph/0201018|bibcode=2002PhRvL..88h1101B|s2cid=41229823}}</ref><ref>{{cite journal|last1=Fimiani|first1=L.|last2=Cook|first2=D. L.|last3=Faestermann|first3=T.|last4=Gómez-Guzmán|first4=J. M.|last5=Hain|first5=K.|last6=Herzog|first6=G.|last7=Knie|first7=K.|last8=Korschinek|first8=G.|last9=Ludwig|first9=P.|last10=Park|first10=J.|last11=Reedy|first11=R. C.|last12=Rugel|first12=G.|title=Interstellar 60Fe on the Surface of the Moon|journal=Physical Review Letters|date=2016|volume=116|issue=15|article-number=151104|doi=10.1103/PhysRevLett.116.151104|pmid=27127953 |bibcode=2016PhRvL.116o1104F}}</ref>

==Research and experiments==
{{See also|Cosmic-ray observatory}}
There are a number of cosmic-ray research initiatives, listed below.

===Ground-based===
{{div col}}
* [[Akeno Giant Air Shower Array]]
* [[Chicago Air Shower Array]]
* [[CHICOS]]
* [[CLOUD]]
* [[Muon Tomography#CRIPT Detector|CRIPT]]
* [[GAMMA]]
* [[GRAPES-3]]
* [[High Altitude Water Cherenkov Experiment|HAWC]]
* [[HEGRA]]
* [[High Energy Stereoscopic System]]
* [[High Resolution Fly's Eye Cosmic Ray Detector]]
* [[IceCube Neutrino Observatory|IceCube]]
* [[KASCADE]]
* [[MAGIC (telescope)|MAGIC]]
* [[MARIACHI]]
* [[Milagro (experiment)|Milagro]]
* [[NMDB]]
* [[Pierre Auger Observatory]]
* [[QuarkNet#Cosmic Ray Studies|QuarkNet]]
* [[Spaceship Earth (detector)|Spaceship Earth]]
* [[Telescope Array Project]]
* [[Tunka experiment]]
* [[VERITAS]]
* [[Washington Large Area Time Coincidence Array]]

{{div col end}}

===Satellite===
{{div col}}
* [[Advanced Composition Explorer|ACE (Advanced Composition Explorer)]]
* [[Alpha Magnetic Spectrometer]]
* ''[[Cassini–Huygens]]''
* [[Fermi Gamma-ray Space Telescope]]
* [[1st High Energy Astronomy Observatory|HEAO 1]], [[Einstein Observatory|HEAO 2]], [[3rd High Energy Astronomy Observatory|HEAO 3]]
* [[Interstellar Boundary Explorer]]
* [[LUCID|Langton Ultimate Cosmic-Ray Intensity Detector]]
* [[Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics|PAMELA]]
* [[Solar and Heliospheric Observatory]]
* ''[[Voyager 1]]'' and ''[[Voyager 2]]''
{{div col end}}

===Balloon-borne===
{{div col}}
* [[Advanced Thin Ionization Calorimeter]]
* [[BESS (experiment)|BESS]]
* [[Cosmic Ray Energetics and Mass]] (CREAM)
* [http://adsabs.harvard.edu/abs/1995psu..reptR....B HEAT (High Energy Antimatter Telescope)]
* [[PERDaix]]
* [http://tiger.gsfc.nasa.gov TIGER] {{Webarchive|url=https://web.archive.org/web/20120503185934/http://tiger.gsfc.nasa.gov/ |date=3 May 2012 }}
* [[TRACER (cosmic ray detector)]]
{{div col end}}

==See also==
{{cmn|
* {{annotated link|Central nervous system effects from radiation exposure during spaceflight}}
* {{annotated link|Cosmic ray visual phenomena}}
* {{annotated link|Environmental radioactivity}}
* {{annotated link|Extragalactic cosmic ray}}
* {{annotated link|Forbush decrease}}
* {{annotated link|Gilbert Jerome Perlow}}
* {{annotated link|Health threat from cosmic rays}}
* {{annotated link|Meter water equivalent}}
* {{annotated link|Oh-My-God particle}}
* {{annotated link|Solar energetic particles}}
* {{annotated link|Track Imaging Cherenkov Experiment}}
* {{annotated link|Ultra-high-energy cosmic ray|abbreviation=UHECR}}
}}

==References==
{{Reflist}}

==Further references==
{{refbegin}}
* {{cite book|title=Introduction to particle and astroparticle physics (multimessenger astronomy and its particle physics foundations)|first1=Alessandro |last1=De Angelis |first2=Mario |last2=Pimenta |year=2018|publisher=Springer|isbn=978-3-319-78181-5|doi=10.1007/978-3-319-78181-5}}
* R.G. Harrison and D.B. Stephenson, Detection of a galactic cosmic ray influence on clouds, Geophysical Research Abstracts, Vol. 8, 07661, 2006 SRef-ID: 1607-7962/gra/EGU06-A-07661
* {{cite journal | last1 = Anderson | first1 = C. D. | last2 = Neddermeyer | first2 = S. H. | date = 1936 | title = Cloud Chamber Observations of Cosmic Rays at 4300 Meters Elevation and Near Sea-Level | url = https://authors.library.caltech.edu/7197/1/ANDpr36.pdf| journal = Phys. Rev. | volume = 50 | issue = 4| pages = 263–271 | doi=10.1103/physrev.50.263|bibcode = 1936PhRv...50..263A }}
* {{cite journal | last1 = Boezio | first1 = M. | display-authors = etal | date = 2000 | title = Measurement of the flux of atmospheric muons with the CAPRICE94 apparatus | journal = Phys. Rev. D | volume = 62 | issue = 3| article-number = 032007 | doi=10.1103/physrevd.62.032007|arxiv = hep-ex/0004014 |bibcode = 2000PhRvD..62c2007B }}
* R. Clay and B. Dawson, Cosmic Bullets, Allen & Unwin, 1997. {{ISBN|1-86448-204-4}}
* T. K. Gaisser, ''Cosmic Rays and Particle Physics'', Cambridge University Press, 1990. {{ISBN|0-521-32667-2}}
* P. K. F. Grieder, Cosmic Rays at Earth: Researcher's Reference Manual and Data Book, Elsevier, 2001. {{ISBN|0-444-50710-8}}
* A. M. Hillas, ''Cosmic Rays'', Pergamon Press, Oxford, 1972 {{ISBN|0-08-016724-1}}
* {{cite journal | last1 = Kremer | first1 = J. | display-authors = etal | date = 1999 | title = Measurement of Ground-Level Muons at Two Geomagnetic Locations | journal = Phys. Rev. Lett. | volume = 83 | issue = 21| pages = 4241–4244 | doi=10.1103/physrevlett.83.4241|bibcode = 1999PhRvL..83.4241K }}
* {{cite journal | last1 = Neddermeyer | first1 = S. H. | last2 = Anderson | first2 = C. D. | date = 1937 | title = Note on the Nature of Cosmic-Ray Particles | doi = 10.1103/physrev.51.884 | journal = Phys. Rev. | volume = 51 | issue = 10| pages = 884–886 |bibcode = 1937PhRv...51..884N | url = https://authors.library.caltech.edu/8618/1/NEDpr37.pdf }}
* M. D. Ngobeni and M. S. Potgieter, Cosmic ray anisotropies in the outer heliosphere, Advances in Space Research, 2007.
* M. D. Ngobeni, Aspects of the modulation of cosmic rays in the outer heliosphere, MSc Dissertation, Northwest University (Potchefstroom campus) South Africa 2006.
* D. Perkins, Particle Astrophysics, Oxford University Press, 2003. {{ISBN|0-19-850951-0}}
* C. E. Rolfs and S. R. William, Cauldrons in the Cosmos, The University of Chicago Press, 1988. {{ISBN|0-226-72456-5}}
* B. B. Rossi, ''Cosmic Rays'', McGraw-Hill, New York, 1964.
* Martin Walt, Introduction to Geomagnetically Trapped Radiation, 1994. {{ISBN|0-521-43143-3}}
* {{cite journal | last1 = Taylor | first1 = M. | last2 = Molla | first2 = M. | date = 2010 | title = Towards a unified source-propagation model of cosmic rays | journal = Publ. Astron. Soc. Pac. | volume = 424 | page = 98 | bibcode = 2010ASPC..424...98T }}
* {{cite journal | last1 = Ziegler | first1 = J. F. | date = 1981 | title = The Background in Detectors Caused By Sea Level Cosmic Rays | journal = Nuclear Instruments and Methods | volume = 191 | issue = 1| pages = 419–424 | doi=10.1016/0029-554x(81)91039-9|bibcode = 1981NIMPR.191..419Z }}
* TRACER Long Duration Balloon Project: the largest cosmic ray detector launched on balloons.
* {{cite journal |last1=Carlson |first1=Per |last2=De Angelis |first2=Alessandro |title=Nationalism and internationalism in science: the case of the discovery of cosmic rays |journal=[[European Physical Journal H]] |date=2011 |doi=10.1140/epjh/e2011-10033-6|bibcode = 2010EPJH...35..309C |arxiv = 1012.5068 |volume=35 |issue=4 |pages=309–329 |s2cid=7635998 }}
{{refend}}

==External links==
{{Commons category|Cosmic rays}}
{{wikiquote}}
*[https://web.archive.org/web/20141116032423/http://www.aspera-eu.org/ Aspera European network portal]
*[https://news.bbc.co.uk/2/hi/science/nature/2868041.stm BBC news, Cosmic rays find uranium, 2003].
*[http://www.mpi-hd.mpg.de/hfm/CosmicRay/Showers.html Introduction to Cosmic Ray Showers] by Konrad Bernlöhr.

{{Radiation}}
{{Molecules detected in outer space}}
{{Portal bar|Physics|Mathematics|Stars|Spaceflight|Solar System|Science}}
{{Authority control}}

{{DEFAULTSORT:Cosmic Ray}}
[[Category:Cosmic rays| ]]
[[Category:Astroparticle physics]]
[[Category:Ionizing radiation]]
[[Category:Stellar phenomena]]
[[Category:Solar phenomena]]
[[Category:Concepts in astronomy]]
[[Category:1912 in science]]