
Cosmic rays are high-energy
proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s and
atomic nuclei
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger-Marsden experiments, Geiger–Marsden gold foil experiment. After the d ...
that move through space at nearly the
speed of light
The speed of light in vacuum
A vacuum is a space
Space is the boundless three-dimensional
Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
. They originate from the
Sun
The Sun is the star
A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

, from outside of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

in our own galaxy, and from distant galaxies. Upon impact with
Earth's atmosphere
The atmosphere of Earth is the layer of gas
Gas is one of the four fundamental states of matter (the others being solid
Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...
, cosmic rays produce
showers of secondary particles, some of which reach the
surface
File:Water droplet lying on a damask.jpg, Water droplet lying on a damask. Surface tension is high enough to prevent floating below the textile.
A surface, as the term is most generally used, is the outermost or uppermost layer of a physical obje ...
; although the bulk is intercepted by the
magnetosphere
In astronomy
Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...
or the
heliosphere
The heliosphere is the vast, bubble-like region of space that surrounds and is created by the Sun. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuou ...
.
Cosmic rays were discovered by
Victor Hess
Victor Franz Hess (24 June 188317 December 1964) was an Austrian-United States, American physicist, and Nobel Prize in Physics, Nobel laureate in physics, who discovered cosmic rays.
Biography
He was born to Vinzenz Hess and Serafine Edle von G ...
in 1912 in balloon experiments, for which he won the 1936
Nobel Prize in Physics
The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for mankind in the field of physics. It is one of the five Nobel Prizes established by the will ...
.
[
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.
Data from the (2013)][
] have been interpreted as evidence that a significant fraction of primary cosmic rays originate from the supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

explosions of stars.[
] Based on observations of neutrino
A neutrino ( or ) (denoted by the Greek letter ) is a fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

s and gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s from blazar
A blazar is an active galactic nucleus
An active galactic nucleus (AGN) is a compact region at the center of a galaxy
A galaxy is a gravitation
Gravity (), or gravitation, is a natural phenomenon by which all things with mass
...
TXS 0506+056
TXS 0506+056 is a very high energy blazar – a quasar with a relativistic jet pointing directly towards Earth – of BL Lac object, BL Lac-type. With a redshift of 0.3365 ± 0.0010, it is about from Earth. Its approximate location o ...
in 2018, active galactic nuclei
An active galactic nucleus (AGN) is a compact region at the center of a galaxy
A galaxy is a gravitation
Gravity (), or gravitation, is a natural phenomenon by which all things with mass
Mass is both a property
Property (''la ...
also appear to produce cosmic rays.[
]
Etymology
The term ''ray'' is somewhat of a misnomer, as cosmic rays were, originally, incorrectly believed to be mostly electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

. In common scientific usage, high-energy particles with intrinsic mass are known as "cosmic" rays, while photons
The photon ( el, φῶς, phōs, light) is a type of elementary particle
In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be ele ...
, which are quanta of electromagnetic radiation (and so have no intrinsic mass) are known by their common names, such as ''gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s'' or ''X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

s'', depending on their photon energy Photon energy is the energy
In physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, i ...
.
Composition
Of primary cosmic rays, which originate outside of Earth's atmosphere, about 99% are the bare nuclei of well-known atoms (stripped of their electron shells), and about 1% are solitary electrons (that is, one type of beta particle
A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β ...
). Of the nuclei, about 90% are simple proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

s (i.e., hydrogen nuclei); 9% are alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Proto ...

s, identical to helium nuclei; and 1% are the nuclei of heavier elements, called HZE ions
HZE ions are the high-energy nuclei
''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic ce ...
.[
] These fractions vary highly over the energy range of cosmic rays. A very small fraction are stable particles of antimatter
In modern physics
Modern physics is a branch of physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...

, such as positron
The positron or antielectron is the antiparticle
s (left) and antiparticles (right). From top to bottom; electron
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is ...

s or antiproton
The antiproton, , (pronounced ''p-bar'') is the antiparticle
s (left) and antiparticles (right). From top to bottom; electron
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than atom
An ...

s. The precise nature of this remaining fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them.
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
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray
Cosmic rays are high-energy proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass sli ...
s have been observed to approach , about 40 million times the energy of particles accelerated by the Large Hadron Collider
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider
A collider is a type of particle accelerator
, a synchrotron collider type particle accelerator at Fermi National Accelerator Laboratory (Fermilab) ...
. One can show that such enormous energies might be achieved by means of the centrifugal mechanism of acceleration
Centrifugal acceleration of astroparticles to relativistic energies might take place in rotating astrophysical objects (see also Fermi acceleration). It is strongly believed that active galactic nuclei and pulsar
Animation of a rotating pulsar ...
in active galactic nuclei
An active galactic nucleus (AGN) is a compact region at the center of a galaxy
A galaxy is a gravitation
Gravity (), or gravitation, is a natural phenomenon by which all things with mass
Mass is both a property
Property (''la ...
. At 50 J, 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 baseball. As a result of these discoveries, there has been interest in investigating cosmic rays of even greater energies. Most cosmic rays, however, do not have such extreme energies; the energy distribution of cosmic rays peaks at .
History
After the discovery of by Henri Becquerel
Antoine Henri Becquerel (; 15 December 1852 – 25 August 1908) was a French engineer
Engineers, as practitioners of engineering
Engineering is the use of scientific principles to design and build machines, structures, and other i ...

in 1896, it was generally believed that atmospheric electricity, ionization
Ionization or ionisation is the process by which an atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
of the air
The atmosphere of Earth is the layer of gas
Gas is one of the four fundamental states of matter
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (ph ...

, was caused only by radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

from radioactive elements in the ground or the radioactive gases or isotopes of radon
Radon is a chemical element
upright=1.0, 500px, The chemical elements ordered by link=Periodic table
In chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that c ...

they produce. 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.
Discovery
In 1909, Theodor Wulf developed an electrometer
An electrometer is an electrical instrument for measuring electric charge
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: ...

, 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
The Eiffel Tower ( ; french: links=yes, tour Eiffel ) is a wrought-iron
Wrought iron is an iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal th ...

than at its base. However, his paper published in ''Physikalische Zeitschrift
''Physikalische Zeitschrift'' (English: ''Physical Journal'') was a German scientific journal
In academic publishing
Academic publishing is the subfield of publishing
Publishing is the activity of making information, literature, music, ...
'' was not widely accepted. In 1911, Domenico Pacini
Domenico Pacini ( Marino, February 20, 1878 – Rome
, established_title = Founded
, established_date = 753 BC
, founder = King Romulus
, image_map = Map of comune of Rome (metropolitan city of Capital Rome, reg ...
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.
In 1912, Victor Hess
Victor Franz Hess (24 June 188317 December 1964) was an Austrian-United States, American physicist, and Nobel Prize in Physics, Nobel laureate in physics, who discovered cosmic rays.
Biography
He was born to Vinzenz Hess and Serafine Edle von G ...
carried three enhanced-accuracy Wulf electrometers 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." In 1913–1914, Werner Kolhörster confirmed Victor Hess's earlier results by measuring the increased ionization enthalpy rate at an altitude of 9 km.]
Hess received the Nobel Prize in Physics
The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for mankind in the field of physics. It is one of the five Nobel Prizes established by the will ...
in 1936 for his discovery.
Identification
Bruno Rossi
Bruno Benedetto Rossi (; ; 13 April 1905 – 21 November 1993) was an Italian experimental physicist. He made major contributions to particle physics
Particle physics (also known as high energy physics) is a branch of physics
...
wrote that:
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 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.
Ernest Rutherford
Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist
A physicist is a scientist
A scientist is a person who conducts scientific research
The sci ...
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".
In the 1920s, the term ''cosmic rays'' was coined by Robert Millikan
Robert Andrews Millikan (March 22, 1868 – December 19, 1953) was an American experimental physicist honored with the Nobel Prize for Physics in 1923 for the measurement of the Elementary charge, elementary electric charge and for his work on ...
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
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

s were produced in the atmosphere by Compton scattering
Compton scattering, discovered by Arthur Holly Compton
Arthur Holly Compton (September 10, 1892 – March 15, 1962) was an American physicist who won the Nobel Prize in Physics in 1927 for his 1923 discovery of the Compton effect, whic ...

of gamma rays. But then, sailing from Java
Java ( id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 147.7 million people, Java is the world's List of ...

to the Netherlands in 1927, Jacob Clay
Jacob Clay () (1882–1955) was a prominent Dutch people, Dutch physicist who first suggested and provided evidence that cosmic rays are charged particles.
Early life
Clay was born "Jacob Claij" in Berkhout on 18 January 1882 as the son of Pieter ...

found evidence, 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. Charged particles of such high energy could not possibly be produced by photons from Millikan's proposed interstellar fusion process.
In 1930, Bruno Rossi
Bruno Benedetto Rossi (; ; 13 April 1905 – 21 November 1993) was an Italian experimental physicist. He made major contributions to particle physics
Particle physics (also known as high energy physics) is a branch of physics
...
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". Three independent experiments 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
The muon (; from the Greek alphabet, Greek letter mu (letter), mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 ''e'' and a spin-½, spin of 1/2, but with a much greater ma ...

s. 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 particle
Alpha particles, also called alpha rays or alpha radiation, consist of two proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Proto ...

s) and 1% are nuclei of heavier elements such as carbon, iron, and lead.
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
A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles o ...

s 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." 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.
Soviet physicist Sergey Vernov was the first to use radiosonde
A radiosonde is a battery-powered telemetry
Telemetry is the or other data at remote points and their automatic to receiving equipment () for monitoring. The word is derived from the roots ''tele'', "remote", and ''metron'', "measure". ...

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
A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles o ...

s in an anti-coincidence circuit to avoid counting secondary ray showers.
Homi J. Bhabha
Homi Jehangir Bhabha (30 October 1909 – 24 January 1966) was an Indian nuclear physics, nuclear physicist, :Institute directors, founding director, and professor of physics at the Tata Institute of Fundamental Research (TIFR). Colloquialism, ...
derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering
In quantum electrodynamics, Bhabha scattering is the electron-positron scattering process:
::e^+ e^- \rightarrow e^+ e^-
There are two leading-order Feynman diagrams contributing to this interaction: an annihilation process and a scattering proces ...
. His classic paper, jointly with Walter Heitler
Walter Heinrich Heitler (; 2 January 1904 – 15 November 1981) was a German physicist
A physicist is a scientist
A scientist is a person who conducts scientific research
The scientific method is an Empirical evidence, empirical me ...
, 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.
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
Massachusetts Institute of Technology (MIT) is a private
Private or privates may refer to:
Music
* "In Private
"In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, aft ...
. The experiment employed eleven arranged within a circle 460 metres in diameter on the grounds of the Agassiz Station of the Harvard College Observatory
The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University
Harvard University is a Private university, private Ivy League research ...
. 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 pampa
The Pampas (from the qu, pampa, meaning "plain") are fertile South America
South America is a entirely in the and mostly in the , with a relatively small portion in the . It can also be described as the southern of a single conti ...
s of Argentina by an international consortium of physicists. The project was first led by James Cronin
James Watson Cronin (September 29, 1931 – August 25, 2016) was an American particle physicist.
Cronin was born in Chicago, Illinois
(''City in a Garden''); I Will
, image_map =
, map_caption = Interactive maps ...
, winner of the 1980 Nobel Prize in Physics
The Nobel Prize in Physics is a yearly award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions for mankind in the field of physics. It is one of the five Nobel Prizes established by the will ...
from the University of Chicago
The University of Chicago (UChicago) is a private
Private or privates may refer to:
Music
* "In Private
"In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, after an abse ...
, and Alan Watson of the University of Leeds
The University of Leeds is a public research university in Leeds, West Yorkshire, England. It was established in 1874 as the Yorkshire College of Science. In 1884 it merged with the Leeds School of Medicine (established 1831) and was renamed Yorks ...
, 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. The results are expected to have important implications for particle physics and cosmology, due to a theoretical Greisen–Zatsepin–Kuzmin limit
The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. The limit is (50 EeV), or about 8 joules (the ener ...
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
The Big Bang theory
A theory is a rational
Rationality is the quality or state of being rational – that is, being based on or agreeable to reason
Reason is the capacity of consciously making sense of things, applying logic
...

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 (>50MeV photons) were finally discovered in the primary cosmic radiation by an MIT experiment carried on the OSO-3 satellite in 1967. 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.
Sources
Early speculation on the sources of cosmic rays included a 1934 proposal by Baade and Zwicky suggesting cosmic rays originated from supernovae. A 1948 proposal by Horace W. Babcock suggested that magnetic variable stars could be a source of cosmic rays. Subsequently, Sekido ''et al.'' (1951) identified the Crab Nebula
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus
Taurus is Latin for 'bull' and may refer to:
* Taurus (constellation), one of the constellations of the zodiac
* Taurus (mythology), one of two Greek mythological characters named ...

as a source of cosmic rays. Since then, a wide variety of potential sources for cosmic rays began to surface, including supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

e, active galactic nuclei
An active galactic nucleus (AGN) is a compact region at the center of a galaxy
A galaxy is a gravitation
Gravity (), or gravitation, is a natural phenomenon by which all things with mass
Mass is both a property
Property (''la ...
, quasar
A quasar (; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus
An active galactic nucleus (AGN) is a compact region at the center of a galaxy
A galaxy is a gravitation
Gravity () ...

s, and gamma-ray burst
In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the brightest and most energetic electromagnetic events known to occur in the universe. Bursts can last f ...
s.
Later experiments have helped to identify the sources of cosmic rays with greater certainty. In 2009, a paper presented at the International Cosmic Ray ConferenceThe International Cosmic Ray Conference, or ICRC, is a physics conference organized biennially by thCommission C4 (Astroparticle Physics) of the International Union of Pure and Applied Physics (IUPAP) since 1947, where physicists from the whole world ...
by scientists at the Pierre Auger Observatory
The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray
Cosmic rays are high-ene ...
in Argentina showed ultra-high energy cosmic rays originating from a location in the sky very close to the radio galaxy
Radio galaxies and their relatives, radio-loud quasar
A quasar (; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus
An active galactic nucleus (AGN) is a compact region at the center of ...
Centaurus A
Centaurus A (also known as NGC 5128 or Caldwell 77) is a galaxy in the constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal ...

, although the authors specifically stated that further investigation would be required to confirm Centaurus A as a source of cosmic rays. 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 × 10−6× erg
The erg is a unit of energy equal to 10−7joule
The joule ( ; symbol: J) is a SI derived unit, derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work (physics), work done on) an objec ...
·cm−2 on the flux of cosmic rays from gamma-ray bursts.
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
The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory
The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory ...
. This analysis, however, was disputed in 2011 with data from PAMELA, which revealed that "spectral shapes of ydrogen and helium nucleiare different and cannot be described well by a single power law", suggesting a more complex process of cosmic ray formation. In February 2013, though, research analyzing data from ''Fermi
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian
Italian may refer to:
* Anything of, from, or related to the country and nation of Italy
** Italians, an ethnic group or simply a citizen of the Italian Republic
** Italia ...

'' revealed through an observation of neutral pion decay that supernovae were indeed a source of cosmic rays, with each explosion producing roughly 3 × 1042 – 3 × 1043 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. 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
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's Physica ...
in the arrival directions of the highest energy cosmic rays. 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
Solar energetic particles (SEP) are high-energy particles coming from the Sun. They were first observed in the early 1940s. They consist of protons
A proton is a subatomic particle, symbol or , with a positive electric charge
Electric char ...

, 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.
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 particle
Alpha particles, also called alpha rays or alpha radiation, consist of two proton
A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Proto ...

s (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, ]lepton
In particle physics, a lepton is an elementary particle of half-integer spin (spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-lik ...

s, and hadron
In particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
s, such as electron
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

s, positrons, muon
The muon (; from the Greek alphabet, Greek letter mu (letter), mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 ''e'' and a spin-½, spin of 1/2, but with a much greater ma ...

s, and pion
In particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that departmen ...

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
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

and sometimes even the Milky Way
The Milky Way is the galaxy
A galaxy is a gravitation
Gravity (), or gravitation, is a natural phenomenon by which all things with mass
Mass is both a property
Property (''latin: Res Privata'') in the Abstract and con ...

. 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%. The remaining fraction is made up of the other heavier nuclei that are typical nucleosynthesis end products, primarily lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: the ...

, beryllium
Beryllium is a chemical element
In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

, and boron
Boron is a chemical element
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

. These nuclei appear in cosmic rays in much greater abundance (≈1%) than in the solar atmosphere, where they are only about 10 as abundant as helium
Helium (from el, ἥλιος, helios
Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

. Cosmic rays composed of charged nuclei heavier than helium are called HZE ions
HZE ions are the high-energy nuclei
''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic ce ...
. Due to the high charge and heavy nature of HZE ions, their contribution to an astronaut's radiation dose
Ionizing radiation (ionising radiation) consists of subatomic particles or electromagnetic wave
Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between elect ...
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
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: the ...

, beryllium
Beryllium is a chemical element
In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

and boron
Boron is a chemical element
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

in a process termed cosmic ray spallation
Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis
Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nucl ...
. Spallation is also responsible for the abundances of scandium
Scandium is a chemical element
upright=1.0, 500px, The chemical elements ordered by link=Periodic table
In chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science tha ...

, titanium
Titanium is a chemical element
In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

, vanadium
Vanadium is a chemical element
upright=1.0, 500px, The chemical elements ordered by link=Periodic table
In chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science tha ...

, and manganese
Manganese is a chemical element
In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical e ...

ion
An ion () is an atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s in cosmic rays produced by collisions of iron and nickel nuclei with interstellar matter
In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmi ...
.
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
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
The Alpha Magnetic Spectrometer (AMS-02) is a particle physics
Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various ty ...
(''AMS-02'') on board the International Space Station
The International Space Station (ISS) is a Modular design, modular space station (habitable satellite, artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (Uni ...

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. 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 . 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. These results on interpretation have been suggested to be due to positron production in annihilation events of massive dark matter
Dark matter is a hypothetical form of matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

particles.
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.
There is no evidence of complex antimatter atomic nuclei, such as antihelium
In modern physics, antimatter is defined as matter that is composed of the antiparticles (or "partners") of the corresponding subatomic particle, particles of "ordinary" matter. Minuscule numbers of antiparticles are generated daily at partic ...
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 on STS-91
STS-91 was the final Space Shuttle
The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. NASA, National Aeronautics and Space Administration (NA ...
in June 1998. By not detecting any antihelium at all, the ''AMS-01'' established an upper limit of for the antihelium to helium flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

ratio.
Secondary cosmic rays
When cosmic rays enter the Earth's atmosphere
The atmosphere of Earth is the layer of gas
Gas is one of the four fundamental states of matter (the others being solid
Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...

, they collide with atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s and molecule
A molecule is an electrically
Electricity is the set of physical phenomena associated with the presence and motion
Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position
In physics, motion is the phenomenon ...

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
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

, protons, alpha particles, pions, muons, electrons, neutrino
A neutrino ( or ) (denoted by the Greek letter ) is a fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

s, and neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s. 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
In particle physics
Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas partic ...

s such as positive or negative pion
In particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that departmen ...

s and kaon
In particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

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
A cloud chamber, also known as a Wilson cloud chamber, is a particle detector
In experimental and applied particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics (from grc, φυσική ...

s, bubble chamber
A bubble chamber is a vessel filled with a superheating, superheated transparency (optics), transparent liquid (most often liquid hydrogen) used to detect electric charge, electrically charged particles moving through it. It was invented in 1952 ...
s, 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
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
emissions by gamma-ray telescope. These are distinguished from radioactive decay processes by their higher energies above about 10 MeV.
Cosmic-ray flux
The flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

of incoming cosmic rays at the upper atmosphere is dependent on the solar wind
The solar wind is a stream of charged particle
In physics
Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

, the Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field
A magnetic field is a vector field
In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. Fo ...
, and the energy of the cosmic rays. At distances of ≈94 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
In geography
Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

, longitude
Longitude (, ) is a geographic coordinate
A geographic coordinate system (GCS) is a coordinate system associated with position (geometry), positions on Earth (geographic position). A GCS can give positions:
*as Geodetic coordinates, ...

, and .
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 and are inferred from lower-energy radiation reaching the ground.
::
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.
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 eV/cm3, which is comparable to the energy density of visible starlight at 0.3 eV/cm3, the galactic magnetic field energy density (assumed 3 microgauss) which is ≈0.25 eV/cm3, or the cosmic microwave background
The cosmic microwave background (CMB, CMBR), in Big Bang
The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
(CMB) radiation energy density at ≈0.25 eV/cm3.
Detection methods
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
The International Space Station (ISS) is a modular
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity i ...

, 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 tracks developed by Robert Fleischer, P. Buford Price, and Robert M. Walker for use in high-altitude balloons. In this method, sheets of clear plastic, like 0.25 mm Lexan
Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transpar ...

polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or ionization
Ionization or ionisation is the process by which an atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
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
Sodium hydroxide, also known as lye
A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali
In chemistry
Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

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 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
Nuclear fission is a reaction
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

.
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 air Cherenkov telescopes. Most state-of-the-art EAS arrays employ plastic scintillator
A scintillator is a material that exhibits scintillation, the property of luminescence
Luminescence is spontaneous emission of light
Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum ...

s. Also water (liquid or frozen) is used as a detection medium through which particles pass and produce Cherenkov radiation to make them detectable. 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
A cloud chamber, also known as a Wilson cloud chamber, is a particle detector
In experimental and applied particle physics
Particle physics (also known as high energy physics) is a branch of physics
Physics (from grc, φυσική ...

s 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
A bubble chamber is a vessel filled with a superheating, superheated transparency (optics), transparent liquid (most often liquid hydrogen) used to detect electric charge, electrically charged particles moving through it. It was invented in 1952 ...
s, can be used to detect cosmic ray particles.
More recently, the CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "see-moss"), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of metal–oxide–semiconductor field-effect transistor
The metal–oxide–se ...
devices in pervasive smartphone
A smartphone is a portable device
A mobile device (or handheld computer) is a computer
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can per ...

cameras have been proposed as a practical distributed network to detect air showers from ultra-high-energy cosmic rays. The first app
App or apps may refer to:
Computing
* Application software
Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typically to be used b ...
to exploit this proposition was the CRAYFIS (Cosmic RAYs Found in Smartphones) experiment. In 2017, the CREDO (Cosmic Ray Extremely Distributed Observatory) Collaboration 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. 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 air Cherenkov telescope, designed to detect low-energy (<200 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
The speed of light in vacuum
A vacuum is a space
Space is the boundless three-dimensional
Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
in their medium, the atmosphere. 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. 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 unstable isotopes, such as carbon-14
Carbon-14 (14C), or radiocarbon, is a radioactive isotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ...

, in the Earth's atmosphere through the reaction:
: n + 14N → p + 14C
Cosmic rays kept the level of carbon-14
Carbon-14 (14C), or radiocarbon, is a radioactive isotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ...

in the atmosphere roughly constant (70 tons) for at least the past 100,000 years, until the beginning of above-ground nuclear weapons testing in the early 1950s. This fact is used in radiocarbon dating
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material
Organic matter, organic material, or natural organic matter refers to the large source of ...
.
;Reaction products of primary cosmic rays, radioisotope half-lifetime, and production reaction
Role in ambient radiation
Cosmic rays constitute a fraction of the annual radiation exposure of human beings on the Earth, averaging 0.39mSv out of a total of 3mSv per year (13% of total background) for the Earth's population. However, the background radiation from cosmic rays increases with altitude, from 0.3mSv per year for sea-level areas to 1.0mSv 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 2.2mSv of extra radiation each year due to cosmic rays, nearly doubling their total exposure to ionizing radiation.
:Figures are for the time before the Fukushima Daiichi nuclear disaster
The Fukushima nuclear disaster was a 2011 nuclear accident
to human
, a Japanese nuclear plant with seven units, the largest single nuclear power station in the world, was completely shut down for 21 months following an earthquake in 2007. S ...
. Human-made values by UNSCEAR are from the Japanese National Institute of Radiological Sciences, which summarized the UNSCEAR data.
Effect on electronics
Cosmic rays have sufficient energy to alter the states of circuit components in electronic
Electronic may refer to:
*Electronics
Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow b ...
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit
200px, A circuit built on a printed circuit board (PCB).
An electronic circuit is composed of indiv ...

s, causing transient errors to occur (such as corrupted data in electronic memory devices or incorrect performance of CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit
File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB).
An electronic circuit is composed of ...

s) often referred to as "soft error
In electronics
Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplifica ...
s". This has been a problem in electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than ...
at extremely high-altitude, such as in satellite
In the context of spaceflight
Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight ...

s, but with transistor
upright=1.4, gate
Candi bentar, a typical Indonesian gate that is often found on the islands of Java">Indonesia.html" ;"title="Candi bentar, a typical Indonesia">Candi bentar, a typical Indonesian gate that is often found on the islands o ...

s becoming smaller and smaller, this is becoming an increasing concern in ground-level electronics as well. Studies by IBM
International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

in the 1990s suggest that computers typically experience about one cosmic-ray-induced error per 256 megabytes of RAM
Random-access memory (RAM; ) is a form of computer memory
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
per month. To alleviate this problem, the Intel Corporation
Intel Corporation is an American multinational corporation
A multinational company (MNC) is a corporate
A corporation is an organization—usually a group of people or a company
A company, abbreviated as co., is a Legal personalit ...
has proposed a cosmic ray detector that could be integrated into future high-density microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip ...

s, allowing the processor to repeat the last command following a cosmic-ray event. ECC memory
Error correction code memory (ECC memory) is a type of computer data storage
File:Maxell DVD-RW 4.7GB crop 20051120.jpg, A spindle of DVD-RWs
Computer data storage is a technology consisting of computer components and Data storage device, ...
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
The Airbus A330 is a wide-body aircraft
A wide-body aircraft, also known as a twin-aisle aircraft, is an with a wide enough to accommodate two passenger s with seven or more seats abreast. The typical diameter is . In the typical wide- ...

airliner to twice 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.
In August 2020 scientists reported that ionizing radiation from environmental radioactive materials and cosmic rays may substantially limit the coherence
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a derive ...
times of qubit
In quantum computing
Quantum computing is a type of computation
Computation is any type of calculation
A calculation is a deliberate process that transforms one or more inputs into one or more results. The term is used in a variety of sens ...
s if they aren't shielded adequately which may be critical for realizing fault-tolerant superconducting quantum computer
Quantum computing is a type of computation
Computation is any type of that includes both al and non-arithmetical steps and which follows a well-defined model (e.g. an ).
Mechanical or electronic devices (or, , people) that perform computatio ...

s in the future.
Significance to aerospace travel
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
''Voyager 2'' is a space probe
A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'' space probe was credited to a single 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.
On 31 May 2013, NASA scientists reported that a possible manned mission to Mars may involve a greater than previously believed, based on the amount of detected by the RAD on the Mars Science Laboratory
Mars Science Laboratory (MSL) is a robotic
Robotics is an interdisciplinarity, interdisciplinary field that integrates computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robot ...
while traveling from the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

to Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

in 2011–2012.[
]
Flying high, passengers and crews of jet airliner
A jet airliner or jetliner is an airliner
An airliner is a type of aircraft
An aircraft is a vehicle or machine that is able to fly
Flies are insect
Insects or Insecta (from Latin
Latin (, or , ) is a classical languag ...
s are exposed to at least 10 times the cosmic ray dose that people at sea level
Mean sea level (MSL) (often shortened to sea level) is an average
In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in th ...

receive. Aircraft flying polar route
A polar route is an aircraft route across the uninhabited polar ice cap regions. The term "polar route" was originally applied to great circle navigation routes between Europe
Europe is a continent
A continent is one of several larg ...
s near the geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the surface of Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surfa ...
s are at particular risk.
Role in lightning
Cosmic rays have been implicated in the triggering of electrical breakdown in lightning
Lightning is a naturally occurring electrostatic discharge
Electrostatic discharge (ESD) is the sudden flow of electricity
Electricity is the set of physical phenomena associated with the presence and motion
Image:Leaving Yongsan ...

. 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.
Postulated role in climate change
A role for cosmic rays in climate was suggested by Edward P. Ney
Edward Purdy Ney (October 28, 1920 – July 9, 1996) was an American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the United States
The United States of Americ ...
in 1959 and by Robert E. Dickinson in 1975. 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. The researchers suggest that this and gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
bombardments deriving from local supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last stellar evolution, evolutionary stages of a massive star or when a ...

e could have affected cancer and mutation rates, and might be linked to decisive alterations in the Earth's climate, and to the Extinction event, mass extinctions of the Ordovician.
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. 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. 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; another example is "incorrect handling of the physical data" resulting in graphs that do not show the correlations they claim to show. Despite Svensmark's assertions, galactic cosmic rays have shown no statistically significant influence on changes in cloud cover, and have been demonstrated in studies to have no causal relationship to changes in global temperature.
Possible mass extinction factor
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.
Research and experiments
There are a number of cosmic-ray research initiatives, listed below.
Ground-based
* 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
The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray
Cosmic rays are high-ene ...
* QuarkNet#Cosmic Ray Studies, QuarkNet
* Spaceship Earth (detector), Spaceship Earth
* Telescope Array Project
* Tunka experiment
* VERITAS
* Washington Large Area Time Coincidence Array
Satellite
* Advanced Composition Explorer, ACE (Advanced Composition Explorer)
* Alpha Magnetic Spectrometer
The Alpha Magnetic Spectrometer (AMS-02) is a particle physics
Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various ty ...
* 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
* PAMELA
* Solar and Heliospheric Observatory
* Voyager 1 and Voyager 2
''Voyager 2'' is a space probe
A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
Balloon-borne
* Advanced Thin Ionization Calorimeter
* BESS (experiment), BESS
* Cosmic Ray Energetics and Mass (CREAM)
HEAT (High Energy Antimatter Telescope)
* PERDaix
TIGER
* TRACER (cosmic ray detector)
See also
References
Further references
*
* 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
*
*
* R. Clay and B. Dawson, Cosmic Bullets, Allen & Unwin, 1997.
* T. K. Gaisser, ''Cosmic Rays and Particle Physics'', Cambridge University Press, 1990.
* P. K. F. Grieder, Cosmic Rays at Earth: Researcher's Reference Manual and Data Book, Elsevier, 2001.
* A. M. Hillas, ''Cosmic Rays'', Pergamon Press, Oxford, 1972
*
*
* 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.
* C. E. Rolfs and S. R. William, Cauldrons in the Cosmos, The University of Chicago Press, 1988.
* B. B. Rossi, ''Cosmic Rays'', McGraw-Hill, New York, 1964.
* Martin Walt, Introduction to Geomagnetically Trapped Radiation, 1994.
*
*
* TRACER Long Duration Balloon Project: the largest cosmic ray detector launched on balloons.
*
External links
Aspera European network portal
Animation about cosmic rays on astroparticle.org
Helmholtz Alliance for Astroparticle Physics
Particle Data Group review of Cosmic Rays
by C. Amsler et al., Physics Letters B667, 1 (2008).
by Konrad Bernlöhr.
BBC news, Cosmic rays find uranium, 2003
BBC news, Rays to nab nuclear smugglers, 2005
BBC news, Physicists probe ancient pyramid (using cosmic rays), 2004
Shielding Space Travelers
by Eugene Parker.
(From NASA's Cosmicopia)
Review of Cosmic Rays
7 October 2005, at Science@NASA
*[https://www.youtube.com/watch?v=xJtSWdT598k video of Muon detector in use at Smithsonian Air and Space Museum]
Dr. Lothar Frey "Cosmic rays and electronic devices"
(YouTube Video) SpaceUp, SpaceUp Stuttgart 2012
ARMAS, Real-time cosmic-ray radiation measurements at aviation altitudes
*
{{DEFAULTSORT:Cosmic Ray
Cosmic rays,
Astroparticle physics
Ionizing radiation
Stellar phenomena
Solar phenomena
Concepts in astronomy