Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90 percent of all the incoming cosmic ray particles are protons, about 9 percent are helium nuclei (alpha particles) and about 1 percent are electrons. Note that the term "ray" is a misnomer, as cosmic ray particles arrive individually, not in the form of a ray or beam of particles.
Cosmic rays originate from a wide variety of sources—ranging from energetic processes on the Sun all the way to the farthest reaches of the visible universe. Most cosmic rays possess kinetic energies between 107 electron volts (eV) and 1010 eV. The range of energies, however, is much wider and depends on the source of the particles. For example, those originating from the Sun have energies of 104-105 eV (1.6-16 fJ) per particle. At the other extreme, some cosmic rays have energies that exceed 1020 eV, far higher than the 1012 to 1013 eV that human-made particle accelerators can produce. There has been interest in investigating cosmic rays of even greater energies.
Cosmic rays form a fraction of the annual radiation that people on Earth are exposed to. For astronauts, however, the risks of exposure are much higher, and it is therefore important to assess these risks. Cosmic rays are thought to trigger electrical breakdown in lightning. Their role in climate change, however, is disputed.
Cosmic ray sources
Most cosmic rays originate from extra-solar sources within earth's own galaxy, such as rotating neutron stars, supernovae, and black holes. However, the fact that some cosmic rays have extremely high energies provides evidence that at least some must be of extra-galactic origin (for example, from radio galaxies and quasars); the local galactic magnetic field would not be able to contain particles with such a high energy. The origin of cosmic rays with energies up to 1014 eV can be accounted for in terms of shock-wave acceleration in supernova shells. The origin of cosmic rays with energy greater than 1014 eV remains unknown; however, a large collaborative experiment at the Pierre Auger Observatory is underway to try to answer this question.
Observations have shown that cosmic rays with an energy above 10 GeV (10 x 109 eV) approach the Earth’s surface isotropically (equally from all directions); it has been hypothesized that this is not due to an even distribution of cosmic ray sources, but instead is due to galactic magnetic fields causing cosmic rays to travel in spiral paths. This limits cosmic ray’s usefulness in positional astronomy, as they carry no information of their direction of origin. At energies below 10 GeV, there is a directional dependence, due to the interaction of the charged component of the cosmic rays with the Earth's magnetic field.
Solar cosmic rays
Solar cosmic rays are cosmic rays that originate from the Sun, with relatively low energy (10-100 keV, or 1.6-16 fJ, per particle). The average composition is similar to that of the Sun itself.
The name, "solar cosmic ray," itself is a misnomer because the term cosmic implies that the rays are from the cosmos and not the Solar System, but it has stuck. The misnomer arose because there is continuity in the energy spectra, that is, the flux of particles as a function of their energy, because the low-energy solar cosmic rays fade more or less smoothly into the galactic ones as one looks at increasingly higher energies. Until the mid-1960s, the energy distributions were generally averaged over long time intervals, which also obscured the difference. Later, it was found that solar cosmic rays vary widely in their intensity and spectrum, increasing in strength after some solar events, such as solar flares.
Furthermore, an increase in the intensity of solar cosmic rays is followed by a decrease in all other cosmic rays, called the Forbush decrease, after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind, with its entrained magnetic field sweeping some of the galactic cosmic rays outward, away from the Sun and Earth. The overall or average rate of Forbush decreases tends to follow the 11-year sunspot cycle, but individual events are tied to events on the Sun, as explained above.
There are further differences between cosmic rays of solar and galactic origin. In particular, galactic cosmic rays show an enhancement of heavy elements such as calcium, iron, and gallium, as well as cosmically rare light elements such as lithium and beryllium. The latter are produced by the cosmic ray spallation (fragmentation) of heavy nuclei due to collisions in transit from the distant sources to the solar system.
Galactic cosmic rays
Galactic cosmic rays are high-energy charged particles that enter the Solar System from the outside the system. They are composed of protons, electrons, and fully ionized nuclei of light elements.
Most galactic cosmic rays have energies too low to penetrate the Earth's atmosphere, and the radii of their helical trajectories in the Earth's magnetic field tends to channel them to the poles. In this respect, these galactic cosmic rays are exactly like the charged particles that make up the solar wind. When they strike the atmosphere, they can create large showers of secondary particles, including exotic ones such as muons, and these secondary particles are what can be detected at the Earth's surface.
In addition, many galactic cosmic rays have extremely high energy, leading to the inference that they must have originated in very energetic processes. Some are believed to have been accelerated by the shockwaves of supernovae. Some galactic cosmic rays have energies so high that no known physical process could have created them.
Very high-energy cosmic rays can penetrate the Earth's atmosphere, and the radii of their helical trajectories are thousands of kilometers, so they are not as effectively channeled by the Earth's magnetic field.
Galactic cosmic rays are among the most important barriers to plans for interplanetary travel by astronauts.
Extragalactic cosmic rays
Extragalactic cosmic rays are very-high-energy particles that flow into our Solar System from beyond our galaxy. The energies these particles possess are in excess of 1015 eV.
Little is known about the origins of extragalactic cosmic rays. Partially, this can be attributed to a lack of statistics: The amount of cosmic rays reaching the Earth's surface originating from extragalactic sources, is about 1 particle per square meter per year.
There are many ideas about which processes may be responsible for cosmic rays with such high energies. In the "bottom-up" approach, cosmic rays gain more and more energy through electromagnetic processes. Bouncing randomly back and forth in the shock waves of some violent object allows some of the particles to gain energy. Eventually, they may build up enough speed to escape from the remnant. Possible sites typically include gamma ray bursts and active galactic nuclei.
There are many more possible sources of cosmic rays that scientists are considering. These include nearby galaxies, colliding galaxy systems, accretion ﬂow shocks to clusters of galaxies, and more exotic processes from the very early universe, such as the decay of superheavy particles trapped in the galactic halo, or topological defects.
Ultra-high-energy cosmic rays
An ultra-high-energy cosmic ray (UHECR) is a cosmic ray (subatomic particle) that appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen-Zatsepin-Kuzmin limit.
The Oh-My-God particle (a play on the nickname "God particle" for the Higgs boson) is the nickname given to a particle observed on the evening of October 15, 1991, over Dugway Proving Grounds, Utah, estimated to have an energy of approximately 3 × 1020 eV, equivalent to about 50 joules. In other words, it was a subatomic particle with macroscopic kinetic energy equal to that of a baseball (140 g) moving at about 27 m/s (60 mph). These very high energy cosmic rays, however, are very rare.
It was most likely a proton traveling with velocity almost equal to the speed of light, and its observation was a shock to astrophysicists. If it were a proton, its speed would have been approximately (1−(5 × 10−24)) c; after traveling one year, the particle would be only 46 nanometers behind a photon that left at the same time.
Since the first observation, by the University of Utah's Fly's Eye Cosmic Ray Detector, at least fifteen similar events have been recorded, confirming the phenomenon. The source of such high energy particles remains a mystery, especially since interactions with blue-shifted cosmic microwave background radiation limit the distance that these particles can travel before losing energy (the Greisen-Zatsepin-Kuzmin limit).
Because of its energy, the Oh-My-God particle would have experienced very little influence from cosmic electromagnetic and gravitational fields, and so its trajectory should be easily calculable. However, nothing of note was found in the estimated direction of its origin.
At a January 12, 2005, conference of the American Astronomical Society, particle physicist Glennys Farrar presented a paper tracing five similar very-high-energy cosmic rays, all of which were detected between 1993 and 2003, to a pair of colliding galaxy clusters 450 million light-years from Earth. Farrar speculated that the clusters' powerful magnetic fields could become warped in the collision, accelerating charged particles to the extreme energies astronomers have observed.
Anomalous cosmic rays
Anomalous cosmic rays (ACRs) are cosmic rays with unexpectedly low energies. They are thought to be created near the edge of earth's solar system, in the heliosheath, the border region between the heliosphere and the interstellar medium. When electrically neutral atoms are able to enter the heliosheath (being unaffected by its magnetic fields) subsequently become ionized, they are thought to be accelerated into low-energy cosmic rays by the solar wind's termination shock, which marks the inner edge of the heliosheath. It is also possible that high energy galactic cosmic rays which hit the shock front of the solar wind near the heliopause might be decelerated, resulting in their transformation into lower-energy anomalous cosmic rays.
The Voyager 1 space probe crossed the termination shock on December 16, 2004, according to papers published in the journal Science. Readings showed particle acceleration, but not of the kind that generates ACRs. It is unclear at this stage (September 2005) if this is typical of the termination shock (requiring a major rethink of the origin of ACRs), or a localized feature of that part of the termination shock that Voyager 1 passed through. Voyager 2 is expected to cross the termination shock during or after 2008, which will provide more data.
Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extra-solar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact with interstellar matter to create secondary cosmic rays. The sun also emits low energy cosmic rays associated with solar flares. The exact composition of primary cosmic rays, outside the Earth’s atmosphere, is dependent on which part of the energy spectrum is observed. However, in general, almost 90 percent of all the incoming cosmic rays are protons, about 9 percent are helium nuclei (alpha particles), and about 1 percent are electrons. The remaining fraction is made up of the other heavier nuclei which are abundant end products of star’s nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not abundant nuclear synthesis end products or products of the Big Bang, primarily lithium, beryllium, and boron. These light nuclei appear in cosmic rays in much greater abundance (about 1:100 particles) than in solar atmospheres, where their abundance is about 10-7 that of helium.
This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termed cosmic ray spallation), into lithium, beryllium, and boron. It is found that the energy spectra of Li, Be, and B falls off somewhat steeper than that of carbon or oxygen, indicating that less cosmic ray spallation occurs for the higher energy nuclei presumably due to their escape from the galactic magnetic field. Spallation is also responsible for the abundances of Sc, Ti, V, and Mn elements in cosmic rays, which are produced by collisions of Fe and Ni nuclei with interstellar matter.
In the past, it was believed that the cosmic ray flux has remained fairly constant over time. Recent research has, however, produced evidence for 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.
The flux (flow rate) of cosmic rays incident on the Earth’s upper atmosphere is modulated (varied) by two processes; the sun’s solar wind and the Earth's magnetic field. Solar wind is expanding magnetized plasma generated by the sun, which has the effect of decelerating the incoming particles as well as partially excluding some of the particles with energies below about 1 GeV. The amount of solar wind is not constant due to changes in solar activity over its regular eleven-year cycle. Hence, the level of modulation varies in autocorrelation with solar activity. Also, the Earth's magnetic field deflects some of the cosmic rays, which is confirmed by the fact that the intensity of cosmic radiation is dependent on latitude, longitude, and azimuth. The cosmic flux varies from eastern and western directions due to the polarity of the Earth’s geomagnetic field and the positive charge dominance in primary cosmic rays; this is termed the east-west effect. The cosmic ray intensity at the equator is lower than at the poles as the geomagnetic cutoff value is greatest at the equator. This can be understood by the fact that charged particle tend to move in the direction of field lines and not across them. This is the reason the Aurorae occur at the poles, since the field lines curve down towards the Earth’s surface there. Finally, the longitude dependence arises from the fact that the geomagnetic dipole axis is not parallel to the Earth’s rotation axis.
This modulation, which describes the change in the interstellar intensities of cosmic rays as they propagate in the heliosphere, is highly energy and spatial dependent, and it is described by the Parker's Transport Equation in the heliosphere. At large radial distances, far from the Sun ~94 AU, there exists the region where the solar wind undergoes a transition from supersonic to subsonic speeds, called the solar wind termination shock. The region between the termination shock and the heliopause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays and it decreases their intensities at lower energies by about 90 percent, indicating that it is not only the Earth's magnetic field that protects it from cosmic ray bombardment.
From a modeling point of view, there is a challenge in determining the Local Interstellar spectra (LIS), due to large adiabatic energy changes these particles experience owing to the diverging solar wind in the heliosphere. However, significant progress has been made in the field of cosmic ray studies with the development of an improved state-of-the-art 2D numerical model that includes the simulation of the solar wind termination shock, drifts and the heliosheath coupled with fresh descriptions of the diffusion tensor (Langner et al. 2004). But challenges also exist because the structure of the solar wind and the turbulent magnetic field in the heliosheath is not well understood indicating the heliosheath as the region unknown beyond. With lack of knowledge of the diffusion coefficient perpendicular to the magnetic field our knowledge of the heliosphere and from the modeling point of view is far from complete. There exist promising theories, like ab initio approaches, but the drawback is that such theories produce poor compatibility with observations (Minnie, 2006), indicating their failure in describing the mechanisms influencing the cosmic rays in the heliosphere.
The nuclei that make up cosmic rays are able to travel from their distant sources to the Earth because of the low density of matter in space. Nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric gases. These collisions, in a process known as a shower, result in the production of many pions and kaons, unstable mesons which quickly decay into muons. Because muons do not interact strongly with the atmosphere and because of the relativistic effect of time dilation many of these muons are able to reach the surface of the Earth. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as bubble chambers or scintillation detectors. If several muons are observed by separated detectors at the same instant it is clear that they must have been produced in the same shower event.
Interaction with the Earth's atmosphere
When cosmic ray particles enter the Earth’s atmosphere, they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of lighter particles, a so-called air shower. The general idea is shown in the figure which shows a cosmic ray shower produced by a high energy proton of cosmic ray origin striking an atmospheric molecule.
This image is a simplified picture of an air shower: In reality, the number of particles created in an air shower event can reach in the billions, depending on the energy of the primary particle. All of the produced particles stay within about one degree of the primary particle's path. Typical particles produced in such collisions are charged mesons (for example, positive and negative pions and kaons); one common collision is:
Cosmic rays are also responsible for the continuous production of a number of unstable isotopes in the Earth’s atmosphere, such as carbon-14, via the reaction:
Cosmic rays have kept the level of carbon-14 in the atmosphere roughly constant (70 tons) for at least the past 100,000 years. This an important fact used in radiocarbon dating, which is commonly used by archaeologists to date ancient artifacts and fossils.
Research and experiments
There are a number of cosmic ray research initiatives. These include, but are not limited to:
- Alpha Magnetic Spectrometer
- Pierre Auger Observatory
- Spaceship Earth
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 (isotopes of radon) they produce. Measurements of ionization rates at increasing heights above the ground during the decade from 1900 to 1910 showed a decrease that could be explained as due to absorption of the ionizing radiation by the intervening air. Then, in 1912, Victor Hess carried three Wulf electrometers (a device to measure the rate of ion production inside a hermetically sealed container) to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. He concluded "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." In 1913-14, Werner Kolhörster confirmed Victor Hess' results by measuring the increased ionization rate at an altitude of 9 km. Hess received the Nobel Prize in Physics in 1936, for his discovery of what came to be called "cosmic rays."
For many years it was generally believed that cosmic rays were high-energy photons (gamma rays) with some secondary electrons produced by Compton scattering of the gamma rays. Then, during the decade from 1927 to 1937, a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positively charged particles, and the secondary radiation observed at ground level is composed primarily of a "soft component" of electrons and photons and a "hard component" of penetrating particles, muons. The muon was initially believed to be the unstable particle predicted by Hideki Yukawa in 1935, in his theory of the nuclear force. Experiments proved that the muon decays with a mean life of 2.2 microseconds into an electron and two neutrinos, but that it does not interact strongly with nuclei, so it could not be the Yukawa particle. The mystery was solved by the discovery in 1947, of the pion, which is produced directly in high-energy nuclear interactions. It decays into a muon and one neutrino with a mean life of 0.0026 microseconds. The pion to muon to electron decay sequence was observed directly in a microscopic examination of particle tracks in a special kind of photographic plate, called a nuclear emulsion that had been exposed to cosmic rays at a high-altitude mountain station. In 1948, observations with nuclear emulsions carried by balloons to near the top of the atmosphere by Gottlieb and Van Allen showed that the primary cosmic particles are mostly protons with some helium nuclei (alpha particles) and a small fraction heavier nuclei.
In 1934, Bruno Rossi reported an observation of near-simultaneous discharges of two Geiger counters widely separated in a horizontal plane during a test of equipment he was using in a measurement of the so-called east-west effect. 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. Unfortunately, he did not have the time to study this phenomenon more closely." In 1937, Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that extensive particle showers are generated by high-energy primary cosmic-ray particles that interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, photons, and muons that reach ground level.
Homi Bhabha derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering. His classic paper, jointly with W. 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. In 1938, Bhabha concluded that observations of the properties of such particles would lead to the straightforward experimental verification of Albert Einstein's theory of relativity.
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. The experiment employed eleven scintillation detectors arranged within a circle 460 meters 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 (past the GZK cutoff, beyond which very few cosmic rays should be observed). 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. 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.
Three varieties of neutrino are produced when the unstable particles produced in cosmic ray showers decay. Since neutrinos interact only weakly with matter most of them simply pass through the Earth and exit the other side. They very occasionally interact, however, and these atmospheric neutrinos have been detected by several deep underground experiments. The Super-Kamiokande in Japan provided the first convincing evidence for neutrino oscillation, in which one flavor of neutrino changes into another. The evidence was found in a difference in the ratio of electron neutrinos to muon neutrinos depending on the distance they have traveled through the air and earth.
Role in ambient radiation
Cosmic rays constitute a fraction of the annual radiation exposure of human beings on earth. For example, the average radiation exposure in Australia is 0.3 mSv due to cosmic rays, out of a total of 2.3 mSv.
Significance to space travel
Understanding the effects of cosmic rays on the body will be vital for assessing the risks of space travel. R.A. Mewaldt estimated humans unshielded in interplanetary space receive annually roughly 400 to 900 mSv (compared to 2.4 mSv on Earth) and that a 30 month Mars mission might expose astronauts to 460 mSv (at Solar Maximum) to 1140 mSv (at Solar Minimum). These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements for Low Earth Orbit activities.
Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel via the Forbush decrease effect. Coronal mass ejections (CMEs) can temporarily lower the local cosmic ray levels, and radiation from CMEs is easier to shield against than cosmic rays.
Role in lightning
Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It has been proposed (Gurevich and Zybin, Physics Today, May 2005, "Runaway Breakdown and the Mysteries of Lightning") that essentially all lightning is triggered through a relativistic process, "runaway breakdown," seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.
Role in climate change
Whether cosmic rays have any role in climate change is disputed. Different groups have made different arguments for the role of cosmic ray forcing in climate change.
Shaviv et. al. have argued that galactic cosmic ray (GCR) climate signals on geological time scales are attributable to changing positions of the galactic spiral arms of the Milky Way, and that Cosmic Ray Flux variability is the most dominant climate driver over these time periods.
They also argue that GCR flux variability plays an important role in climate variability over shorter time scales, though the relative contribution of anthropogenic factors in relation to GCR flux presently is a matter of continued debate. Because there remains some uncertainty about which GCR energies are the most important drivers of cloud cover variation (if any), and because of the paucity of historical data on cosmic ray flux at various ranges of energies, controversies remain.
What is a mechanism whereby GCR flux variability may affect global climate? Henrik Svensmark et al. have argued that solar variations modulate the cosmic ray signal seen at the earth and that this would affect cloud formation and hence climate. Cosmic rays have been experimentally determined to be able to produce ultra-small aerosol particles, orders of magnitude smaller than cloud condensation nuclei (CCN). But the steps from this to modulation of cloud formation and thence to be a contributor of global warming have not been established. The analogy is with the Wilson cloud chamber, however acting on a global scale, where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of CCN. But unlike a cloud chamber, where the air is carefully purified, the real atmosphere always has many CCN naturally. Various proposals have been made for the exact mechanism by which cosmic rays might affect clouds, including Ion Mediated Nucleation, and through an indirect effect on current flow density in the Global electric circuit (Tinsley 2000, and F. Yu 1999). Claims have been made of identification of GCR climate signals in atmospheric parameters such as high latitude precipitation (Todd & Kniveton), and Svensmark's annual cloud cover variations, which were said to be correlated to GCR variation.
That Svensmark's work can be extrapolated to suggest any meaningful connection with global warming is disputed.
At the time we pointed out that while the experiments were potentially of interest, they are a long way from actually demonstrating an influence of cosmic rays on the real world climate, and in no way justify the hyperbole that Svensmark and colleagues put into their press releases and more 'popular' pieces. Even if the evidence for solar forcing were legitimate, any bizarre calculus that takes evidence for solar forcing of climate as evidence against greenhouse gases for current climate change is simply wrong. Whether cosmic rays are correlated with climate or not, they have been regularly measured by the neutron monitor at Climax Station (Colorado) since 1953 and show no long term trend. No trend = no explanation for current changes.
Cosmic rays and fiction
Because of the metaphysical connotations of the word "cosmic," the very name of these particles enables their misinterpretation by the public, giving them an aura of mysterious powers. Were they merely referred to as "high-speed protons and atomic nuclei," this might not be so.
In fiction, cosmic rays have been used as a catchall, mostly in comics (notably the Marvel Comics group, the Fantastic Four), as a source for mutation and therefore the powers gained by being bombarded with them.
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