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{{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 fluxflux(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 particles or clusters of particles (primarily represented by protons 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 our own galaxy,BOOK, Sharma, Shatendra, Atomic and Nuclear Physics, 2008, Pearson Education India, 978-81-317-1924-4, 478, Cosmic rays were discovered by Victor Hess in 1912 in balloon experiments, for which he was awarded the 1936 Nobel Prize in Physics.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.BOOK, Cilek, Vaclav, 2009, Earth System: History and Natural Variability, I, 165, Cosmic Influences on the Earth, Eolss Publishers, 978-1-84826-104-4,weblink Data from the Fermi Space Telescope (2013)JOURNAL, Ackermann, M., Ajello, M., Allafort, A., Baldini, L., Ballet, J., Barbiellini, G., Baring, M.G., Bastieri, D., Bechtol, K., Bellazzini, R., Blandford, R.D., Bloom, E.D., Bonamente, E., Borgland, A.W., Bottacini, E., Brandt, T.J., Bregeon, J., Brigida, M., Bruel, P., Buehler, R., Busetto, G., Buson, S., Caliandro, G.A., Cameron, R.A., Caraveo, P.A., Patrizia A. Caraveo, Casandjian, J.M., Cecchi, C., Celik, O., Charles, E., Chaty, S., 6, 15 February 2013, Science (journal), Science, 339, 6424, 807–811, Detection of the characteristic pion decay-signature in supernova remnants, 10.1126/science.1231160, 1302.3307, 2013Sci...339..807A, 23413352, 29815601, {{better source needed|date=September 2023}} Based on observations of neutrinos and gamma rays from blazar TXS 0506+056 in 2018, active galactic nuclei also appear to produce cosmic rays.JOURNAL , JOURNAL, Aartsen, Mark, IceCube Collaboration, 12 July 2018, Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert, Science, 361, 6398, 147–151, 10.1126/science.aat2890, 0036-8075, 30002248, 2018Sci...361..147I, 1807.08794, 133261745,

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.WEB, Christian, Eric, Are cosmic rays electromagnetic radiation?, NASA,weblinkweblink" title="web.archive.org/web/20000531200225weblink">weblink dead, 31 May 2000, 11 December 2012,

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 protons (i.e., hydrogen nuclei); 9% are alpha particles, identical to helium nuclei; and 1% are the nuclei of heavier elements, called HZE ions.WEB, What are cosmic rays?, NASA, Goddard Space Flight Center,weblink 31 October 2012, dead,weblink" title="web.archive.org/web/20121028154200weblink">weblink 28 October 2012, WEB, mirror copy, also archived,weblinkweblink" title="web.archive.org/web/20160304080140weblink">weblink 4 March 2016, Upon striking the atmosphere, cosmic rays violently burst atoms into other bits of matter, producing large amounts of pions and muons (produced from the decay of charged pions, which have a short half-life) as well as neutrinos.WEB, Resnick, Brian, Extremely powerful cosmic rays are raining down on us. No one knows where they come from, Vox Media, July 25, 2019,weblink 2022-12-14, The neutron composition of the particle cascade increases at lower elevations, reaching between 40% and 80% of the radiation at aircraft altitudes.JOURNAL, Sovilj MP, Vuković B, Stanić D, 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, Arhiv Za Higijenu Rada I Toksikologiju, 71, 2, 152–157, 2020, 10.2478/aiht-2020-71-3403, 7968484, 32975102, 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.CERNweblink 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 rays have been observed to approach {{nowrap|3 × {{10^|20}} eV }}NEWS, Nerlich, Steve, 12 June 2011, Astronomy without a telescope – 'Oh-my-God' particles, Universe Today,weblink 17 February 2013, (This is slightly greater than 21 million times the design energy of particles accelerated by the Large Hadron Collider, {{convert|14|TeV|eV|abbr=~|lk=on}}.WEB, LHC: The guide, FAQ: Facts and figures, 2021, 3, European Organization for Nuclear Research (CERN), Large Hadron Collider,weblink 9 October 2022, ) 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}},MAGAZINE, Gaensler, Brian, November 2011, Extreme speed, COSMOS, 41,weblink dead,weblink" title="archive.today/20130407234845weblink">weblink 7 April 2013, the highest-energy ultra-high-energy cosmic rays (such as the 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.JOURNAL, Anchordoqui, L., Paul, T., Reucroft, S., Swain, J., 2003, Ultrahigh energy cosmic rays: The state of the art before the Auger Observatory, International Journal of Modern Physics A, 18, 13, 2229–2366, 10.1142/S0217751X03013879, hep-ph/0206072, 2003IJMPA..18.2229A, 119407673, 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}}.WEB, Nave, Carl R., Cosmic rays, HyperPhysics, Georgia State University, Physics and Astronomy Department,weblink 17 February 2013,

History

After the discovery of radioactivity by Henri Becquerel in 1896, it was generally believed that atmospheric electricity, ionization of the air, was caused only by radiation from radioactive elements in the ground or the radioactive gases or isotopes of radon they produce.BOOK, Malley, Marjorie C., 25 August 2011, Radioactivity: A History of a Mysterious Science, 78–79, Oxford University Press, 9780199766413,weblink 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.BOOK, John, North, 15 July 2008, Cosmos: An Illustrated History of Astronomy and Cosmology, 686, University of Chicago Press, 9780226594415,weblink

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.JOURNAL, Wulf, Theodor, Beobachtungen über die Strahlung hoher Durchdringungsfähigkeit auf dem Eiffelturm, Physikalische Zeitschrift, 1910, 11, 811–813,weblink Observations of radiation of high penetration power at the Eiffel tower, German, 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.JOURNAL, Pacini, D., 1912, La radiazione penetrante alla superficie ed in seno alle acque, Il Nuovo Cimento, 3, 1, 93–100, 10.1007/BF02957440, 1002.1810, 1912NCim....3...93P, 118487938, : Translated with commentary inJOURNAL, de Angelis, A., 2010, Penetrating Radiation at the Surface of and in Water, Il Nuovo Cimento, 3, 1, 93–100, 1002.1810, 1912NCim....3...93P, 10.1007/BF02957440, 118487938, In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometersWEB,weblink Nobel Prize in Physics 1936 – Presentation Speech, Nobelprize.org, 10 December 1936, 27 February 2013, to an altitude of 5,300 metres in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. 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. 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."JOURNAL, Hess, V.F., Victor Francis Hess, 1912, Ãœber Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten, On observations of penetrating radiation during seven free balloon flights, de, Physikalische Zeitschrift, 13, 1084–1091, 1808.02927, In 1913–1914, Werner Kolhörster confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km.JOURNAL, Kolhörster, Werner, Messungen der durchdringenden Strahlung im Freiballon in größeren Höhen, Physikalische Zeitschrift, 1913, 14, 1153–1156,weblink Measurements of the penetrating radiation in a free balloon at high altitudes, German, JOURNAL, Kolhörster, W., Messungen der durchdringenden Strahlungen bis in Höhen von 9300 m., Verhandlungen der Deutschen Physikalischen Gesellschaft, 1914, 16, 719–721,weblink Measurements of the penetrating radiation up to heights of 9300 m., German, (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.WEB, Hess, V.F., Victor Francis Hess, 1936, The Nobel Prize in Physics 1936, The Nobel Foundation,weblink 11 February 2010, WEB, Hess, V.F., Victor Francis Hess, 1936, Unsolved Problems in Physics: Tasks for the Immediate Future in Cosmic Ray Studies, Nobel Lectures, The Nobel Foundation,weblink 11 February 2010, (File:Hessballon.jpg|thumb|Hess lands after his balloon flight in 1912.)

Identification

Bruno Rossi wrote in 1964: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.BOOK, Rossi, Bruno Benedetto, Cosmic Rays, New York, McGraw-Hill, 1964, 978-0-07-053890-0, 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".JOURNAL, Geiger, H., Rutherford, Lord, Regener, E., Lindemann, F.A., Wilson, C.T.R., Chadwick, J., Gray, L.H., Tarrant, G.T.P., Dobson, G.M.B., 6, 1931, Discussion on Ultra-Penetrating Rays, Proceedings of the Royal Society of London A, 132, 819, 331, 1931RSPSA.132..331G, 10.1098/rspa.1931.0104, free, 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 electrons were produced in the atmosphere by Compton scattering of gamma rays. In 1927, while sailing from Java to the Netherlands, Jacob Clay found evidence,JOURNAL, Clay, J., 1927, Penetrating Radiation, Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam, 30, 9–10, 1115–1127,weblinkweblink" title="web.archive.org/web/20160206055736weblink">weblink 2016-02-06, live, 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, Bothe and Kolhörster discovered charged cosmic-ray particles that could penetrate 4.1 cm of gold.JOURNAL, Bothe, Walther, Werner Kolhörster, November 1929, Das Wesen der Höhenstrahlung, Zeitschrift für Physik, 56, 11–12, 751–777, 10.1007/BF01340137, 1929ZPhy...56..751B, 123901197, 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".JOURNAL, On the Magnetic Deflection of Cosmic Rays, Rossi, Bruno, Physical Review, August 1930, 36, 3, 606, 10.1103/PhysRev.36.606, 1930PhRv...36..606R, Three independent experimentsJOURNAL, The Azimuthal Asymmetry of the Cosmic Radiation, Johnson, Thomas H., Physical Review, May 1933, 43, 10, 834–835, 10.1103/PhysRev.43.834, 1933PhRv...43..834J, JOURNAL, A Positively Charged Component of Cosmic Rays, Alvarez, Luis, Compton, Arthur Holly, Physical Review, May 1933, 43, 10, 835–836, 10.1103/PhysRev.43.835, 1933PhRv...43..835A, JOURNAL, Directional Measurements on the Cosmic Rays Near the Geomagnetic Equator, Rossi, Bruno, Physical Review, May 1934, 45, 3, 212–214, 10.1103/PhysRev.45.212, 1934PhRv...45..212R, 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 muons. In 1948, observations with nuclear emulsions 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.JOURNAL, Evidence for Heavy Nuclei in the Primary Cosmic radiation, Freier, Phyllis, Physical Review, July 1948, 74, 2, 213–217, 10.1103/PhysRev.74.213, Lofgren, E., Ney, E., Oppenheimer, F., Bradt, H., Peters, B., 1948PhRv...74..213F, etal, JOURNAL, Investigation of the Primary Cosmic Radiation with Nuclear Photographic Emulsions, Freier, Phyllis, Physical Review, December 1948, 74, 12, 1828–1837, 10.1103/PhysRev.74.1828, Peters, B., 1948PhRv...74.1828B, etal, 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 counters was larger than the expected accidental rate. In his report on the experiment, Rossi wrote "... 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."JOURNAL, Misure sulla distribuzione angolare di intensita della radiazione penetrante all'Asmara, Rossi, Bruno, Ricerca Scientifica, 1934, 5, 1, 579–589, In 1937, 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.{{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=.}}Soviet physicist Sergei Vernov was the first to use radiosondes 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 counters in an anti-coincidence circuit to avoid counting secondary ray showers.BOOK, J.L. DuBois, R.P. Multhauf, C.A. Ziegler, 2002, The Invention and Development of the Radiosonde,weblinkweblink" title="web.archive.org/web/20110605132224weblink">weblink 2011-06-05, live, Smithsonian Studies in History and Technology, 53, Smithsonian Institution Press, JOURNAL, S. Vernoff, 1935, Radio-Transmission of Cosmic Ray Data from the Stratosphere, Nature, 135, 3426, 1072–1073, 10.1038/1351072c0, 1935Natur.135.1072V, 4132258, 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.JOURNAL, Bhabha, H. J., Heitler, W., The Passage of Fast Electrons and the Theory of Cosmic Showers, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 159, 898, 1937, 432–458, 1364-5021, 10.1098/rspa.1937.0082,weblinkweblink" title="web.archive.org/web/20160102023556weblink">weblink 2016-01-02, live, 1937RSPSA.159..432B, free, JOURNAL, Braunschweig, W., etal, A study of Bhabha scattering at PETRA energies, Zeitschrift für Physik C, 37, 2, 1988, 171–177, 10.1007/BF01579904, 121904361,

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 extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology.JOURNAL, Clark, G., Earl, J., Kraushaar, W., Linsley, J., Rossi, B., Scherb, F., Scott, D., 10.1103/PhysRev.122.637, Cosmic-Ray Air Showers at Sea Level, Physical Review, 122, 2, 637–654, 1961, 1961PhRv..122..637C, The experiment employed eleven 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 1020 eV. A huge air shower experiment called the Auger Project is currently operated at a site on the Pampas 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 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.WEB,weblink The Pierre Auger Observatory, Auger Project, live,weblink 3 September 2018, 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 1020 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.JOURNAL, (none), Kraushaar, W. L., The Astrophysical Journal, 1972, 177, 341, 10.1086/151713, 1972ApJ...177..341K, etal, free, 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.

Modulation

The solar cycle causes variations in the magnetic field of the solar wind through which cosmic rays propagate to Earth.This results in a modulation the arriving fluxes at lower energies, as detected indirectly by the globally distributed neutron monitor network.

Sources

Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and Zwicky suggesting cosmic rays originated from supernovae.JOURNAL, Baade, W., Zwicky, F., 1934, Cosmic rays from super-novae, Proceedings of the National Academy of Sciences of the United States of America, 20, 5, 259–263, 86841, 10.1073/pnas.20.5.259, 16587882, 1934PNAS...20..259B, 1076396, free, A 1948 proposal by Horace W. Babcock suggested that magnetic variable stars could be a source of cosmic rays.JOURNAL, Babcock, H., Magnetic variable stars as sources of cosmic rays, 10.1103/PhysRev.74.489, Physical Review, 74, 4, 489, 1948, 1948PhRv...74..489B, Subsequently, Sekido et al. (1951) identified the Crab Nebula as a source of cosmic rays.JOURNAL, Sekido, Y., Masuda, T., Yoshida, S., Wada, M., The Crab Nebula as an observed point source of cosmic rays, 10.1103/PhysRev.83.658.2, Physical Review, 83, 3, 658–659, 1951, 1951PhRv...83..658S, Since then, a wide variety of potential sources for cosmic rays began to surface, including supernovae, active galactic nuclei, quasars, and gamma-ray bursts.WEB, Gibb, Meredith, Cosmic rays,weblink Imagine the Universe, NASA Goddard Space Flight Center, 17 March 2013, 3 February 2010, (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 A as a source of cosmic rays.CONFERENCE,weblink Correlation of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects in Pierre Auger Observatory Data, 17 March 2013, Hague, J.D., Proceedings of the 31st ICRC, Łódź 2009, July 2009, International Cosmic Ray Conference, Łódź, Poland, 6–9, dead,weblink" title="web.archive.org/web/20130528011624weblink">weblink 28 May 2013, 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 Ã— 10−6× erg·cm−2 on the flux of {{nowrap|1 GeV – 1 TeV}} cosmic rays from gamma-ray bursts.JOURNAL, Hague, J.D., Correlation of the highest energy cosmic rays with nearby extragalactic objects in Pierre Auger Observatory data,weblink Proceedings of the 31st ICRC, Łódź, Poland 2009 – International Cosmic Ray Conference, July 2009, 36–39, 17 March 2013, dead,weblink" title="web.archive.org/web/20130528011624weblink">weblink 28 May 2013, 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.WEB,weblink Source of cosmic rays pinned down, Tech Media Network, Space.com, 25 June 2009, 20 March 2013, Moskowitz, Clara, Clara Moskowitz, This analysis, however, was disputed in 2011 with data from 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.JOURNAL, Adriani, O., Barbarino, G.C., Bazilevskaya, G.A., Bellotti, R., Boezio, M., Bogomolov, E.A., Bonechi, L., Bongi, M., Bonvicini, V., Borisov, 10.1126/science.1199172, S., Bottai, S., Bruno, A., Cafagna, F., Campana, D., Carbone, R., Carlson, P., Casolino, M., Castellini, G., Consiglio, L., de Pascale, M.P., de Santis, C., de Simone, N., di Felice, V., Galper, A.M., Gillard, W., Grishantseva, L., Jerse, G., Karelin, A.V., Koldashov, S.V., Krutkov, S.Y., PAMELA measurements of cosmic-ray proton and helium spectra, Science, 332, 6025, 69–72, 2011, 21385721, 1103.4055, 2011Sci...332...69A, 6, 2108/55474, 1234739, 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 Ã— 1042 – 3 Ã— 1043{{nbsp}}J of cosmic rays.(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.NEWS,weblink Cosmic ray mystery solved, Guardian News and Media Ltd, The Guardian, London, UK, 14 February 2013, 21 March 2013, Jha, Alok, 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.JOURNAL, The Pierre Auger Collaboration, Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8×10{{sup, 18 , 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}} 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 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. Secondary cosmic rays, caused by a decay of primary cosmic rays as they impact an atmosphere, include photons, hadrons, and leptons, such as electrons, positrons, muons, and pions. 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%.WEB, Mewaldt, Richard A., 1996, Cosmic Rays, California Institute of Technology,weblink 26 December 2012, 30 August 2009,weblink" title="web.archive.org/web/20090830191145weblink">weblink dead, 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 in a process termed cosmic ray spallation. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter.JOURNAL, The relative abundances of the elements scandium to manganese in relativistic cosmic rays and the possible radioactive decay of manganese 54, Koch, L., Engelmann, J. J., Goret, P., Juliusson, E., Petrou, N., Rio, Y., Soutoul, A., Byrnak, B., Lund, N., Peters, B., Astronomy and Astrophysics, October 1981, 102, 11, 1981A&A...102L...9K, L9, 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.JOURNAL, L., Accardo, etal, AMS Collaboration, High statistics measurement of the positron fraction in primary cosmic rays of 0.5–500 GeV with the alpha magnetic spectrometer on the International Space Station, Physical Review Letters, 18 September 2014, 113, 12, 121101, 10.1103/PhysRevLett.113.121101, 25279616,weblinkweblink" title="web.archive.org/web/20141017131844weblink">weblink 2014-10-17, live, 2014PhRvL.113l1101A, free, JOURNAL, Schirber, Michael, 2014, Synopsis: More dark matter hints from cosmic rays?, Physical Review Letters, 113, 12, 121102, 10.1103/PhysRevLett.113.121102, 25279617, 1701.07305, 2014PhRvL.113l1102A,weblink 1721.1/90426, 2585508, 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 GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500 GeV, but peaks at energies far higher than electron energies, which peak about 10 GeV.WEB, New results from the Alpha Magnetic$Spectrometer on the International Space Station,weblinkweblink" title="web.archive.org/web/20140923222913weblink">weblink 2014-09-23, live, AMS-02 at NASA, 21 September 2014, These results on interpretation have been suggested to be due to positron production in annihilation events of massive dark matter particles.JOURNAL, Aguilar, M., Alberti, G., Alpat, B., Alvino, A., Ambrosi, G., Andeen, K., Anderhub, H., Arruda, L., Azzarello, P., Bachlechner, A., Barao, F., Baret, B., Barrau, A., Barrin, L., Bartoloni, A., Basara, L., Basili, A., Batalha, L., Bates, J., Battiston, R., Bazo, J., Becker, R., Becker, U., Behlmann, M., Beischer, B., Berdugo, J., Berges, P., Bertucci, B., Bigongiari, G., Biland, A., 6, 2013, 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 GeV, Physical Review Letters, 110, 14, 141102, 25166975, 2013PhRvL.110n1102A, 10.1103/PhysRevLett.110.141102,weblinkweblink 2017-08-13, live, free, 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 GeV, indicating their production in a fundamentally different process from cosmic ray protons, which on average have only one-sixth of the energy.JOURNAL, Secondary antiprotons and propagation of cosmic rays in the Galaxy and heliosphere, Moskalenko, I.V., Strong, A.W., Ormes, J.F., Potgieter, M.S., January 2002, The Astrophysical Journal, 565, 1, 280–296, 10.1086/324402, astro-ph/0106567, 2002ApJ...565..280M, 5863020, 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 × 10−6}} for the antihelium to helium flux ratio.JOURNAL, AMS Collaboration, Aguilar, M., Alcaraz, J., Allaby, J., Alpat, B., Ambrosi, G., Anderhub, H., Ao, L., Arefiev, A., 6, August 2002, The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – Results from the test flight on the space shuttle, Physics Reports, 366, 6, 331–405, 2002PhR...366..331A, 10.1016/S0370-1573(02)00013-3, 2078.1/72661, 122726107, {{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 m below ground, at the Soudan 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 MeV. These are produced by cosmic ray bombardment on its surface.WEB, 1 August 2005, EGRET detection of gamma rays from the Moon,weblink NASA, GSFC, 11 February 2010, }}

Secondary cosmic rays

When cosmic rays enter the Earth's atmosphere, they collide with atoms and molecules, 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 neutrons.BOOK, Morison, Ian, Introduction to Astronomy and Cosmology, 2008, John Wiley & Sons, 978-0-470-03333-3, 198, 2008iac..book.....M, 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 mesons such as positive or negative pions and kaons. 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 chambers, bubble chambers, water-Cherenkov, or 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 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.WEB, Extreme Space Weather Events, National Geophysical Data CenterNational Geophysical Data CenterThe 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 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 acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤ 1 GeV) by about 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 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 planetWEB,weblink How many?, Cosmic rays, Pierre Auger Observatory, Auger.org, 17 August 2012, dead,weblink" title="web.archive.org/web/20121012090629weblink">weblink 12 October 2012, and are inferred from lower-energy radiation reaching the ground.WEB,weblink The mystery of high-energy cosmic rays, Pierre Auger Observatory, Auger.org, 15 July 2015, 8 March 2021,weblink dead,