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Cosmic rays

Electrons and the nuclei of atoms—largely hydrogen—that impinge upon Earth from all directions of space with nearly the speed of light. Before they enter the atmosphere they are typically referred to as primary cosmic rays, to distinguish them from the particles generated by their interaction with the terrestrial atmosphere. Secondary cosmic rays, comprising a large variety of species of charged and neutral particles, cascade down through the atmosphere all the way to the ground and below. Study of cosmic rays at high energy now is often referred to as particle astrophysics.

Cosmic rays are studied for a variety of reasons, not the least of which is a general curiosity over the process by which nature can produce such energetic nuclei. Apart from this, primary cosmic rays provide the only direct sample of matter from far outside the solar system. Measurement of their composition can aid in understanding which properties of the matter making up the solar system are typical of the Milky Way Galaxy as a whole and which may be so atypical as to yield specific clues to the origin of the solar system. Cosmic rays are electrically charged; hence they are deflected by the magnetic fields which are thought to exist throughout the Milky Way Galaxy, and may be used as probes to determine the nature of these fields far from Earth. Outside the solar system the energy contained in the cosmic rays is comparable to that of the magnetic field, so the cosmic rays probably play a major role in determining the structure of the field. Collisions between cosmic rays and the nuclei of the atoms in the tenuous gas which permeates the Milky Way Galaxy change the cosmic-ray composition in a measurable way and produce gamma rays which can be detected at Earth, giving information on the distribution of this gas.  See also: Gamma-ray astronomy

This modern understanding of cosmic rays evolved through a process of discovery which at many times produced seemingly contradictory results, the ultimate resolution of which led to fundamental discoveries in other fields of physics, most notably high-energy particle physics. At the turn of the century several different types of radiation were being studied, and the different properties of each were being determined with precision. One result of many precise experiments was that an unknown source of radiation existed with properties that were difficult to characterize. In 1912 Viktor Hess made a definitive series of balloon flights which showed that this background radiation increased with altitude in a dramatic fashion. Far more penetrating then any other known at that time, this radiation had many other unusual properties and became known as cosmic radiation, because it clearly did not originate in the Earth or from any known properties of the atmosphere.

Unlike the properties of alpha-, beta-, gamma-, and x-radiation, the properties of cosmic radiation are not of any one type of particle, but are due to the interactions of a whole series of unstable particles, none of which was known at that time. The initial identification of the positron, the muon, the π meson or pion, and certain of the K mesons and hyperons were made from studies of cosmic rays.

Thus the term cosmic ray does not refer to a particular type of energetic particle, but to energetic particles being considered in their astrophysical context. The effects of cosmic rays on living cells are discussed in a number of other articles: for example,  See also: Elementary particle; Radiation injury (biology)

Cosmic-ray detection

Cosmic rays are usually detected by instruments which classify each incident particle as to type, energy, and in some cases time and direction of arrival. A convenient unit for measuring cosmic-ray energy is the electronvolt, which is the energy gained by a unit charge (such as an electron) accelerating freely across a potential of 1 V. One electronvolt equals about 1.6 × 10−19 joule. For nuclei it is usual to express the energy in terms of electronvolts per nucleon, since the relative abundances of the different elements are nearly constant as a function of this variable. Two nuclei with the same energy per nucleon have the same velocity.

Flux

The intensity of cosmic radiation is generally expressed as a flux by dividing the average number seen per second by the effective size or “geometry factor” of the measuring instrument. Calculation of the geometry factor requires knowledge of both the sensitive area (in square centimeters) and the angular acceptance (in steradians) of the detector, as the arrival directions of the cosmic rays are randomly distributed to within 1% in most cases. A flat detector of any shape but with area of 1 cm2 has a geometry factor of π cm2 · sr if it is sensitive to cosmic rays entering from one side only. The total flux of cosmic rays in the vicinity of the Earth but outside the atmosphere is about 0.3 nucleus/(cm2 · s · sr) [2 nuclei/(in.2 · s · sr)]. Thus a quarter dollar, with a surface area of 4.5 cm2 (0.7 in.2), lying flat on the surface of the Moon will be struck by 0.3 × 4.5 × 3.14 = 4.2 cosmic rays per second.

Energy spectrum

The flux of cosmic rays varies as a function of energy. This function, called an energy spectrum, may refer to all cosmic rays or to only a selected element or group of elements. Since cosmic rays are continuously distributed in energy, it is meaningless to attempt to specify the flux at any exact energy. Normally an integral spectrum is used, in which the function gives the total flux of particles with energy greater than the specified energy [in particles/(cm2 · s · sr)], or a differential spectrum, in which the function provides the flux of particles in some energy interval (typically 1 MeV/nucleon wide) centered on the specified energy, in particles/[cm2 · s · sr · (MeV/nucleon)]. The basic approach of cosmic-ray research is to measure the spectra of the different components of cosmic radiation and to deduce from them and other observations the nature of the cosmic-ray sources and the details of where the particles travel on their way to Earth and what they encounter on their journey.

Types of detectors

All cosmic-ray detectors are sensitive to moving electrical charges. Neutral particles (neutrons, gamma rays, and neutrinos) are studied by observing charged particles produced in the collision of the neutral primary with some type of target. At low energies the ionization of the matter through which they pass is the principal means of detection. Such detectors include cloud chambers, ion chambers, spark chambers, Geiger counters, proportional counters, scintillation counters, solid-state detectors, photographic emulsions, and chemical etching of certain mineral crystals or plastics in which ionization damage is revealed. The amount of ionization produced by a particle is given by the square of its charge multiplied by a universal function of its velocity, the Bethe-Bloch relation. A single measurement of the ionization produced by a particle is therefore usually not sufficient both to identify the particle and to determine its energy. However, since the ionization itself represents a significant energy loss to a low-energy particle, it is possible to design systems of detectors which trace the rate at which the particle slows down and thus to obtain unique identification and energy measurement.  See also: Gamma-ray detectors; Geiger-Müller counter; Ionization chamber; Junction detector; Particle track etching; Photographic materials; Scintillation counter

At energies above about 500 MeV/nucleon, almost all cosmic rays will suffer a catastrophic nuclear interaction before they slow appreciably. Some measurements are made using massive calorimeters which are designed to trap all of the energy from the cascade of particles which results from such an interaction. More commonly an ionization measurement is combined with measurement of a physical effect that varies in a different way with mass, charge, and energy. Cerenkov detectors and the deflection of the particles in the field of large superconducting magnets (or the magnetic field of the Earth itself) provide the best means of studying energies up to a few hundred gigaelectronvolts per nucleon. Detectors of x-ray transition radiation are useful for measuring composition at energies up to a few thousand GeV per nucleon. Transition radiation detectors are also used to study electrons having energies of 10–200 GeV which, because of their lower rest mass, are already much more relativistic than protons of the same energies.  See also: Cerenkov radiation; Superconducting devices; Transition radiation detectors

Above about 1014 eV, direct detection of individual particles is no longer practical, simply because they are so rare. Such particles are studied by observing the large showers of secondaries they produce in Earth's atmosphere. These showers are detected either by counting the particles which survive to strike ground-level detectors or by looking at the flashes of light the showers produce in the atmosphere with special telescopes and photomultiplier tubes. It is not possible to directly determine what kind of particle produces any given shower. Because of the extreme energies involved, which can be measured with fair accuracy and have been seen as high as 1020 eV (16 J), most of the collision products travel in the same direction as the primary and at essentially the speed of light. This center of intense activity has typical dimensions of only a few tens of meters, allowing it to be tracked (with sensitive instruments) like a miniature meteor across the sky before it hits the Earth at a well-defined location. In addition to allowing determination of the direction from which each particle came, the development of many such showers through the atmosphere may be studied statistically to gain an idea of whether the primaries are protons or heavier nuclei. The main idea behind these studies is that a heavy nucleus, in which the energy is initially shared among several neutrons and protons, will cause a shower that starts higher in the atmosphere and develops more regularly than a shower which has the same total energy but is caused by a single proton.  See also: Particle detector; Photomultiplier

Atmospheric cosmic rays

The primary cosmic-ray particles coming into the top of the terrestrial atmosphere make inelastic collisions with nuclei in the atmosphere. The collision cross section is essentially the geometrical cross section of the nucleus, of the order of 10−26 cm2 (10−27 in.2). The mean free path for primary penetration into the atmosphere is given in Table 1. (Division by the atmospheric density in g/cm3 gives the value of the mean free path in centimeters.)

When a high-energy nucleus collides with the nucleus of an air atom, a number of things usually occur. Rapid deceleration of the incoming nucleus leads to production of pions with positive, negative, or neutral charge; this meson production is closely analogous to the generation of x-rays, or bremsstrahlung, produced when a fast electron is deflected by impact with the atoms in a metal target. The mesons, like the bremsstrahlung, come off from the impact in a narrow cone in the forward direction. Anywhere from 0 to 30 or more pions may be produced, depending upon the energy of the incident nucleus. The ratio of neutral to charged pions is about 0.75. A few protons and neutrons (in about equal proportions) may be ejected with energies up to a few GeV. They are called knock-on protons and neutrons.  See also: Bremsstrahlung; Meson; Nuclear reaction

A nucleus struck by a proton or neutron with energy greater than approximately 300 MeV may have its internal forces momentarily disrupted so that some of its nucleons are free to leave with their original nuclear kinetic energies of about 10 MeV. The nucleons freed in this fashion appear as protons, deuterons, tritons, alpha particles, and even somewhat heavier clumps, radiating outward from the struck nucleus. In photographic emulsions the result is a number of short prongs radiating from the point of collision, and for this reason it is called a nuclear star.

All these protons, neutrons, and pions generated by collision of the primary cosmic-ray nuclei with the nuclei of air atoms are the first stage in the development of the secondary cosmic-ray particles observed inside the atmosphere. Since several secondary particles are produced by each collision, the total number of energetic particles of cosmic-ray origin will at first increase with depth, even while the primary density is decreasing. Since electric charge must be conserved and the primaries are positively charged, positive particles outnumber negative particles in the secondary radiation by a factor of about 1.2. This factor is called the positive excess.

Electromagnetic cascade

Uncharged π0 mesons decay into two gamma rays with a lifetime of about 9 × 10−17 s. The decay is so rapid that π0 mesons are not directly observed among the secondary particles in the atmosphere. The two gamma rays, which together have the rest energy of the π0, about 140 MeV, plus the π0 kinetic energy, each produce a positron-electron pair. Upon passing sufficiently close to the nucleus of an air atom deeper in the atmosphere, the electrons and positrons convert their energy into bremsstrahlung. The bremsstrahlung in turn creates new positron-electron pairs, and so on. This cascade process continues until the energy of the initial π0 has been dispersed into a shower of positrons, electrons, and photons with insufficient individual energies (≤1 MeV) to continue the pair production. The shower, then being unable to reproduce its numbers, is dissipated by ionization of the air atoms. The electrons and photons of such showers are referred to as the soft component of the atmospheric (secondary) cosmic rays, reaching a maximum intensity at an atmospheric depth of 150–200 g/cm2 and then declining by a factor of about 102 down to sea level.  See also: Electron-positron pair production

Muons

The π± mesons produced by the primary collisions have a lifetime about 2.6 × 10−8 s before they decay into muons: π± → μ± + neutrino. With a lifetime of this order a π± possessing enough energy (greater than 10 GeV) to experience significant relativistic time dilatation may exist long enough to interact with the nuclei of the air atoms. The cross section for π± nuclear interactions is approximately the geometrical cross section of the nucleus, and the result of such an interaction is essentially the same as for the primary cosmic-ray protons. Most low-energy π± decay into muons before they have time to undergo nuclear interactions.

Except at very high energy (above 500 GeV), muons interact relatively weakly with nuclei, and are too massive (207 electron masses) to produce bremsstrahlung. They lose energy mainly by the comparatively feeble process of ionizing air atoms as they progress downward through the atmosphere. Because of this ability to penetrate matter, they are called the hard component. At rest their lifetime is 2 × 10−6 s before they decay into an electron or positron and two neutrinos, but with the relativistic time dilatation of their high energy, 5% of the muons reach the ground. Their interaction with matter is so weak that they penetrate deep into the ground, where they are the only charged particles of cosmic-ray origin to be found. At a depth equivalent of 300 m (990 ft) of water the muon intensity has decreased from that at ground level only by a factor of 20; at 1400 m (4620 ft) it has decreased by a factor of 103.

Atmospheric neutrinos

In the late 1990s, detectors became available with sufficient sensitivity to exploit atmospheric neutrinos. Neutrinos of different types are produced in association with muons and electrons, and it is possible to calculate the expected flux of each type with some accuracy. Production of other types of neutrinos is predicted to be quite small. The detected flux of muon neutrinos is significantly lower than the calculation, in analogy with a similar deficit in the neutrino flux from the Sun. Data from the large detector Super Kamiokande in Japan gave the first indication that the atmospheric deficit is due to transformation (known as oscillation) of muon neutrinos into other types of neutrinos. The Sudbury Neutrino Observatory in Canada has confirmed this transformation, demonstrating that the rest mass of the neutrino, while very small, is not zero. Astrophysical consequences of a nonzero rest mass are profound, as a particle with a rest mass interacts gravitationally in a way totally different from that of a particle (such as a photon) with no rest mass. Huge numbers of neutrinos permeate the universe, and details of their gravitational interaction are crucial to the understanding of galaxy formation.  See also: Neutrino

Nucleonic component

The high-energy nucleons—the knock-on protons and neutrons—produced by the primary-particle collisions and a few pion collisions proceed down into the atmosphere. They produce nuclear interactions of the same kind as the primary nuclei, though of course with diminished energies. This cascade process forms the nucleonic component of the secondary cosmic rays.

When nucleon energy falls below about 100 MeV, stars and further knock-ons can no longer be produced. At the same time the protons are rapidly disappearing from the cascade because their ionization losses in the air slow them down before they can make a nuclear interaction. Most of the hadrons in the lower atmosphere are thus neutrons, which are already dominant at 3500 m (11,550 ft), about 300 g/cm2 (4.3 lb/in.2) above sea level, where they outnumber the protons four to one. Thus the final stages of the cascade involve mainly neutrons in a sequence of low-energy interactions which convert them to thermal neutrons (neutrons of kinetic energy of about 0.025 eV) in a path of about 90 g/cm2 (1.3 lb/in.2). These thermal neutrons are readily detected in boron trifluoride (BF3) and helium-3 (3He) counters. The nucleonic component increases in intensity down to a depth of about 120 g/cm2 (1.7 g/cm3), and thereafter declines in intensity, with a mean absorption length of about 200 g/cm2 (2.8 lb/in.2).

The various cascades of secondary particles in the atmosphere are shown schematically in Fig. 1. About 48% of the initial primary cosmic-ray energy goes into charged pions, 25% into neutral pions, 7% into the nucleonic component, and 20% into stars. The nucleonic component is produced principally by the lower-energy (about 5 GeV) primaries. Higher-energy primaries put their energy more into meson production. Hence in the lower atmosphere, a Geiger counter responds mainly to the higher-energy primaries (about 5 GeV) because it counts the muons and electrons, whereas a BF3 counter detecting thermal neutrons responds more to the low-energy primaries.

Fig. 1  Cascade of secondary cosmic-ray particles in the terrestrial atmosphere.

Neutrinos

Cosmic neutrinos, detected for the first time from the explosion of the supernova 1987A, provide confirmation of theoretical calculations regarding the collapse of the cores of massive stars. Although neutrinos are produced in huge numbers (over 1015 passed through a typical human body from this supernova), they interact with matter only very weakly, necessitating a very large detector. Detectors consisting of huge tanks containing hundreds of tons of pure water located deep underground to reduce the background produced by other cosmic rays recorded less than two dozen neutrino events. Still larger detectors, which are under construction in the Antarctic ice and underwater at several locations, will permit observation of more distant supernovae and allow sensitive searches for point sources of high-energy neutrinos. Additionally, by measuring the fraction of non-neutrino-induced events containing multiple muons, these new detectors can investigate the composition of cosmic rays at energies above 1015 eV. Some preliminary measurements indicate that these high-energy cosmic rays may consist primarily of iron nuclei rather than the protons that dominate at lower energies. Much of the interest in the new neutrino observatories derives from the success of the now maturing field of measurement of the flux of solar neutrinos, which is really quite a different problem. With the realization that neutrinos have mass, increasingly precise measurements of the solar neutrino flux, coupled with such techniques as helioseismology, continue to make fundamental contributions to the study of the internal structure of the Sun.  See also: Neutrino astronomy; Solar neutrinos; Supernova

Relation to particle physics

Investigations of cosmic rays continue to make fundamental contributions to particle physics. Neutrino detectors, besides detecting oscillations of atmospheric neutrinos, have set the best limit yet (about 1032 years) on the lifetime of the proton. Cosmic rays remain the only source of particles with energies above 1000 GeV. With the continued increase in the size and sensitivity of detectors, study of cosmic rays should continue to provide the first indications of new physics at ultrahigh energies.  See also: Fundamental interactions; Proton

Geomagnetic effects

The magnetic field of Earth is described approximately as that of a magnetic dipole of strength 8.1 × 1015 weber-meters (8.1 × 1025 gauss · cm3) located near the geometric center of Earth. Near the Equator the field intensity is 3 × 10−5 tesla (0.3 gauss), falling off in space as the inverse cube of the distance to the Earth's center. In a magnetic field which does not vary in time, the path of a particle is determined entirely by its rigidity, or momentum per unit charge; the velocity simply determines how fast the particle will move along this path. Momentum is usually expressed in units of eV/c, where c is the velocity of light, because at high energies, energy and momentum are then numerically almost equal. By definition, momentum and rigidity are numerically equal for singly charged particles. The unit so defined is dimensionally a volt, but the relationship to electric potential is neither obvious nor particularly useful in practice. Table 2 gives examples of these units as applied to different particles with rigidity of 1 gigavolt. This corresponds to an orbital radius in a typical interplanetary (10−9 tesla or 10−5 gauss) magnetic field of approximately 10 times the distance from the Earth to the Moon.  See also: Relativistic electrodynamics

The minimum rigidity of a particle able to reach the top of the atmosphere at a particular geomagnetic latitude is called the geomagnetic cutoff rigidity at that latitude, and its calculation is a complex numerical problem. Fortunately, for an observer near the ground, obliquely arriving secondary particles, produced by the oblique primaries, are so heavily attenuated by their longer path to the ground that it is usually sufficient to consider only the geomagnetic cutoff for vertically incident primaries, which is given in Table 3. Around the Equator, where a particle must come in perpendicular to the geomagnetic lines of force to reach Earth, particles with rigidity less than 10 GV are entirely excluded, though at higher latitudes where entry can be made more nearly along the lines of force, lower energies can reach Earth. Thus, the cosmic-ray intensity is a minimum at the Equator, and increases to its full value at either pole—this is the cosmic-ray latitude effect. Even deep in the atmosphere the variation with latitude is easily detected with BF3 counters (Fig. 2). North of 45° the effect is slight because the additional primaries admitted are so low in energy that they produce few secondaries.

Fig. 2  Latitude variation of the neutron component of cosmic rays in 80°W longitude and at a height corresponding to an atmospheric pressure of 30 kPa (22.5 cm of mercury) in 1948, when the Sun was active, and 1954, when the Sun was deep in a sunspot minimum.

Accurate calculations of the geomagnetic cutoff must consider the deviations of the true field from that of a perfect dipole and the change with time of these deviations. Additionally the distortion of the field by the pressure of the solar wind must often be accounted for, particularly at high latitude. Such corrections vary rapidly with time because of sudden bursts of solar activity and because of the rotation of the Earth. Areas with cutoffs of 400 MV during the day may have no cutoff at all during the night. This day-night effect is confined to particles with energies so low that neither they nor their secondaries reach the ground, and is thus observed only on high-altitude balloons or satellites.

Since the geomagnetic field is directed from south to north above the surface of Earth, the incoming cosmic-ray nuclei are deflected toward the east. Hence an observer finds some 20% more particles incident from the west. This is known as the east-west effect.  See also: Geomagnetism

Solar modulation

Figure 3 presents portions of the proton and alpha-particle spectra observed near the Earth but outside of the magnetosphere in 1973. Below 20 GeV/nucleon the cosmic-ray intensity varies markedly with time. S. Forbush was the first to show that the cosmic-ray intensity was low during the years of high solar activity and sunspot number, which follow an 11-year cycle. This effect is clearly seen in the data of Fig. 2 and has been extensively studied with ground-based and spacecraft instruments. While this so-called solar modulation is now understood in general terms, it has not been calculated in detail, in large part because there are few direct measurements out of the ecliptic plane and in the outer heliosphere.

Fig. 3  Spectra of cosmic-ray protons and helium at Earth and in nearby interstellar space, showing the effect of solar modulation. Observations were made in 1973, when the Sun was quiet.

The primary cause of solar modulation is the solar wind, a highly ionized gas (plasma) which originates from the solar corona and propagates radially from the Sun at a velocity of about 400 km/s (250 mi/s). The wind is mostly hydrogen, with typical density of 5 protons/cm3 (80 protons/in.3). This density is too low for collisions with cosmic rays to be important. Rather, the high conductivity of the medium traps part of the solar magnetic field and carries it outward. The rotation of the Sun and the radial motion of the plasma combine to create the observed archimedean spiral pattern of the average interplanetary magnetic field. Turbulence in the solar wind creates fluctuations in the field which often locally obscure the average direction and intensity. This complex system of magnetic irregularities propagating outward from the Sun deflects and sweeps the low-rigidity cosmic rays out of the solar system.  See also: Solar magnetic field

In addition to the bulk sweeping action, another effect of great importance occurs in the solar wind, adiabatic deceleration. Because the wind is blowing out, only those particles which chance to move upstream fast enough are able to reach Earth. However, because of the expansion of the wind, particles interacting with it lose energy. Thus, particles observed at Earth at 10 MeV/nucleon actually started out at several hundred megaelectronvolts per nucleon in nearby interstellar space, while those with only 100–200 MeV/nucleon initial energy probably never reach Earth at all. This is particularly unfortunate because at these lower energies the variation with energy of nuclear reaction probabilities would allow much more detailed investigation of cosmic-ray history. Changes in the modulation with solar activity are caused by the changes in the pattern of magnetic irregularities rather than by changes in the wind velocity, which are quite small.  See also: Magnetohydrodynamics; Plasma (physics)

Heliosphere

Solar modulation is important in a region around the Sun termed the heliosphere, a large bubble formed in the interstellar medium by the solar wind. The density, and therefore the energy and momentum, of the solar wind drop as the material expands with increasing distance from the Sun, eventually becoming too small to push back the interstellar material. The typical distance to the interface is thought to be approximately 100 AU, but the actual distance in any direction is determined by local variations in both the solar wind and the interstellar medium. (1 AU, or astronomical unit, is the average Earth-Sun separation, 1.49 × 108 km or 9.26 × 107 mi.).

The spacecraft Voyager 1 crossed the termination shock of the solar wind on December 16, 2004, at some 94 astronomical units (AU) or more than 8.7 × 109 miles from the Sun, as evidenced by an abrupt increase in the magnetic field. The termination shock is the innermost, and probably the best-defined, structure in this boundary region. Outside the termination shock several centuries worth of decelerated solar wind are probably piled up, producing a region that is still capable of modulating cosmic-ray intensity. The Sun, carrying the heliosphere with it, is moving through the interstellar medium at approximately 20 km/s (12 mi/s). Eventually all of the solar material blends into the interstellar medium by turbulent interactions. The termination shock had been universally thought to be a prodigious accelerator of particles and Voyager 1 largely confirmed this. At the shock there is a remarkable increase in particle intensity with a distinctive energy spectrum.

Forbush decreases

Apart from the 11-year modulation cycle, there are many different types of cosmic-ray variation associated with irregularities in the solar wind. The most dramatic is the Forbush decrease, wherein worldwide cosmic-ray intensity may drop as much as 20% in one day, followed by a slow recovery lasting many days or even weeks. Most Forbush decreases are associated with severe magnetic disturbances in the solar wind that result from massive ejections of material from the solar corona into interplanetary space. Often these ejections accompany solar flares. When magnetic disturbances encounter the Earth, they can cause geomagnetic storms and other phenomena that are disruptive to human activity. This complex set of interactions has come to be called space weather. Observing changes in cosmic-ray fluxes from several places on Earth simultaneously is one important tool for investigating the interaction of a magnetic disturbance with Earth.  See also: Solar wind; Sun

Composition of cosmic rays

Nuclei ranging from protons to lead have been identified in the cosmic radiation. The relative abundances of the elements ranging up to nickel are shown in Fig. 4, together with the best estimate of the “universal abundances” obtained by combining measurements of solar spectra, lunar and terrestrial rocks, meteorites, and so forth. Most obvious is the similarity between these two distributions. However, a systematic deviation is quickly apparent: the elements lithium-boron and scandium-manganese as well as most of the odd-charged nuclei are vastly overabundant in the cosmic radiation. This effect has a simple explanation: the cosmic rays travel great distances in the Milky Way Galaxy and occasionally collide with atoms of interstellar gas—mostly hydrogen and helium—and fragment. This fragmentation, or spallation as it is called, produces lighter nuclei from heavier ones but does not change the energy/nucleon very much. Thus the energy spectra of the secondary elements are similar to those of the primaries.  See also: Spallation reaction

Fig. 4  Cosmic-ray abundances compared to the universal abundances of the elements. Carbon is set arbitrarily to an abundance of 100 in both cases.

Calculations involving reaction probabilities determined by nuclear physicists show that the overabundances of the secondary elements can be explained by assuming that cosmic rays pass through an average of about 5 g/cm2 (0.07 lb/in.2) of material on their way to Earth. Although an average path length can be obtained, it is not possible to fit the data by saying that all particles of a given energy have exactly the same path length; furthermore, results indicate that higher-energy particles traverse less matter in reaching the solar system, although their original composition seems energy independent.  See also: Elements, cosmic abundance of

When spallation has been corrected for, differences between cosmic-ray abundances and solar-system or universal abundances still remain. The most important question is whether these differences are due to the cosmic rays having come from a special kind of material (such as would be produced in a supernova explosion), or simply to the fact that some atoms might be more easily accelerated than others. It is possible to rank almost all of the overabundances by considering the first ionization potential of the atom and the rigidity of the resulting ion, although this approach gives no prediction of the magnitude of the enhancement. Relative abundances of particles accelerated in solar flares are also far from constant from one flare to the next. Accounting for these abundance variations is one of the most important constraints on models of solar particle acceleration, the exact mechanism of which remains an unsolved problem.  See also: Ionization potential

Isotopes

Much current cosmic-ray research is concentrated on determining isotopic composition of the elements, partly because this is less likely to be changed by acceleration than the elemental composition and thus is more accurately representative of the composition of the source material. As an example, the low-energy helium data in Fig. 3 are not well represented by the calculation. The excess flux, which is referred to as the anomalous component, is nearly all 4He, whereas higher-energy cosmic rays are nearly 10% 3He. A similar enhancement of low-energy nitrogen is pure 14N, while at higher energies nitrogen is half 15N. Measuring isotopes allows conclusive identification of the anomalous component as a sample of originally neutral interstellar material that has been ionized and energized by processes in the solar wind.

Other variations in the isotopic composition are not currently understood. For example, the ratio of 22Ne to 20Ne in the cosmic-ray sources is estimated to be 0.37, while the accepted solar system value for this number is 0.12, which agrees well with the abundances measured in solar-flare particles. However, another direct sample of solar material—the solar wind—has a ratio of 0.08, indicating clearly that the isotopic composition of energetic particles need not reflect that of their source. Conclusions drawn from the observed difference in the solar and cosmic-ray values must be viewed as somewhat tentative until the cause of the variation in the solar material is well understood.  See also: Isotope

Electron abundance

Cosmic-ray electron measurements pose other problems of interpretation, partly because electrons are nearly 2000 times lighter than protons, the next lightest cosmic-ray component. Protons with kinetic energy above 1 GeV are about 100 times as numerous as electrons above the same energy, with the relative number of electrons decreasing slowly at higher energies. But it takes about 2000 GeV to give a proton the same velocity as a 1-GeV electron. Viewed in this way electrons are several thousand times more abundant than protons. (Electrical neutrality of the Milky Way Galaxy is maintained by lower-energy ions which are more numerous than cosmic rays although they do not carry much energy.) It is thus quite possible that cosmic electrons have a different source entirely from the nuclei. It is generally accepted that there must be direct acceleration of electrons, because calculations show that more positrons than negatrons should be produced in collisions of cosmic-ray nuclei with interstellar gas. Measurements show, however, that only 10% of the electrons are positrons. As the number of positrons seen agrees with the calculated secondary production, added confidence is gained in the result that there is indeed an excess of negatrons.  See also: Electron

Electrons are light enough to emit a significant amount of synchrotron radiation as they are deflected by the 10−10-tesla (10−6-gauss) galactic magnetic field. Measurement of this radiation by radio telescopes provides sufficient data for an approximate calculation of the average energy spectrum of electrons in interstellar space and other galaxies. Comparison of spectra of electrons and positrons measured at Earth with those calculated to exist in interstellar space provides the most direct measurement of the absolute amount of solar modulation.  See also: Radio astronomy; Synchrotron radiation

Properties of the energy spectrum

At energies above 1010 eV, the energy spectra of almost all cosmic rays are approximated over many decades by functions in which the flux decreases as the energy raised to some negative, nonintegral power referred to as the spectral index. Such a power-law relationship is of course a straight line when plotted using logarithmic axes. A steep or “soft” (that is, more rapidly falling with increasing energy) spectrum thus has a higher spectral index than a flat or “hard” spectrum. The straight-line regions of the spectra in Fig. 3 correspond to a variation of flux with a spectral index of −2.7. A spectral index of −2.7 provides a good fit with the data up to 1015 eV total energy. Between 1015 and 1019 eV a steeper spectrum, with an index around −3.0, seems to be well established. Above 1019 eV the spectrum hardens once more, returning to an index of about −2.7. The spectral index above 1020 eV has not been determined, because particles are so rare that they are almost never seen, even in detectors which cover several square kilometers and operate for many years. At such high energies, the individual particles are not identified, and changes in the measured-energy spectrum could be the result of composition changes. However, the evidence available indicates that the composition is essentially unchanged.

The Pierre Auger Observatory, which began operation in 2005, has a detection area the size of Rhode Island (over 3000 km2 or 1200 mi2) located in western Argentina's Mendoza Province. The Auger Observatory is a hybrid detector, employing two independent methods to detect and study high-energy cosmic rays. One technique is ground-based and detects high-energy particles through their interaction with water. Each of the 1600 detectors contains 11,000 L (3000 gals) of ultrapure, deionized water. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere.

Age

Another important result which can be derived from detailed knowledge of cosmic-ray isotopic composition is the “age” of cosmic radiation. Certain isotopes are radioactive, such as beryllium-10 (10Be) with a half-life of 1.6 × 106 years. Since beryllium is produced entirely by spallation, study of the relative abundance of 10Be to the other beryllium isotopes, particularly as a function of energy to utilize the relativistic increase in this lifetime, will yield a number related to the average time since the last nuclear collision. Measurements show that 10Be is nearly absent at low energies, yielding an estimate of the age of the cosmic rays of approximately 107 years. An implication of this result is that the cosmic rays propagate in a region in space which has an average density of 0.1–0.2 atom/cm3 (1.5–3 atoms/in.3). This is consistent with some astronomical observations of the immediate solar neighborhood.

Very high energy particles cannot travel long distances in the 2.7 K blackbody-radiation field which permeates the universe. Electrons of 15 GeV energy lose a good portion of their energy in 108 years by colliding with photons via the (inverse) Compton process, yet electrons are observed to energies of 100 GeV and over. A similar loss mechanism becomes effective at approximately 1020 eV for protons. These observations are of course not conclusive, but a safe statement is that a cosmic-ray age of 107 years is consistent with all currently available data.  See also: Compton effect; Cosmic background radiation

Several attempts have been made to measure the constancy of the cosmic-ray flux in time. Variations in 14C production, deduced from apparent deviations of the archeological carbon-dating scale from that derived from studies of tree rings, cover a period of about 103 years. Radioactive 10Be in deep-sea sediments allows studies over 106 years, whereas etching of tracks left by cosmic rays in lunar minerals covers a period of 109 years. None of these methods has ever indicated a variation of more than a factor of 2 in average intensity. There are big differences in these time scales, and the apparent constancy of the flux could be due to averaging over variations which fall in the gaps as far as the time scales are concerned. Nevertheless, the simplest picture seems to be that the cosmic rays are constant in time at an intensity level which is due to a long-term balance between continuous production and escape from the Galaxy, with an average residence time of 107 years.  See also: Cosmogenic nuclide; Radiocarbon dating

Although small, variations in cosmic-ray intensity show systematic patterns over historical and geologic time scales. Some of this variation is undoubtedly due to variability in the magnetic field of the Earth that results in changes in the geomagnetic cutoff. However, over the past several thousand years, when the magnetic field has been reasonably constant, cosmic-ray intensity has shown a distinct anticorrelation with measures of overall global temperature. Low cosmic-ray fluxes, indicative of enhanced solar activity, typically go together with warm periods. The relationship is far from understood, but this is one of many observations that must be considered in the complex, nonlinear problem of global warming.

Origin

Although study of cosmic rays has yielded valuable insight into the structure, operation, and history of the universe, their origin has not been determined. The problem is not so much to devise processes which might produce cosmic rays, but to decide which of many possible processes do in fact produce them.

In general, analysis of the problem of cosmic-ray origin is broken into two major parts: origin in the sense of where the sources are located, whatever they are, and origin in the sense of how the particles are accelerated to such high energies. Of course, these questions can never be separated completely.

Location of sources

It is thought that cosmic rays are produced by mechanisms operating within galaxies and are confined almost entirely to the galaxy of their production, trapped by the galactic magnetic field. The intensity in intergalactic space would only be a few percent of the typical galactic intensity, and would be the result of a slow leakage of the galactic particles out of the magnetic trap. It has not been possible to say much about where the cosmic rays come from by observing their arrival directions at Earth. At lower energies (up to 1015 eV) the anisotropies which have been observed can all be traced to the effects of the solar wind and interplanetary magnetic field. The magnetic field of the Milky Way Galaxy seems to be completely effective in scrambling the arrival directions of these particles.

For several decades in energy above 1015 a smoothly rising anisotropy is measured, ranging from 0.1 to 10%, but the direction of the maximum intensity varies in a nonsystematic way with energy. At these energies, particles have a radius of curvature which is not negligible compared to galactic structures, and thus their arrival direction could be related to where they came from but in a complex way. Above 1019 eV the radius of curvature in the galactic magnetic field becomes comparable to or larger than galactic dimensions, making containment of such particles in the disk of the Milky Way Galaxy impossible. Only a few hundred events greater than 1019 eV have been detected, with no systematic anisotropy. Clustering of events (of marginal statistical significance) in the data of individual observatories has been reported, but so far never confirmed by other detectors. The Pierre Auger Observatory should, over several years, increase the number of events observed by orders of magnitude and may possibly permit statistically definitive determinations of features in the arrival direction distribution. Actual identification of sources, however, will probably come only from detection of neutral radiation (photons and neutrinos) produced in cosmic-ray interactions.

Cerenkov telescopes, such as HESS in Namibia and MAGIC at the Roque de los Muchachos Observatory in the Canary Islands, have produced remarkable images of objects in the light of 1012-eV gamma rays. Supernova remnants have been clearly resolved to show structures within them, and emission from x-ray binary systems has been definitively observed. Several sources unassociated with known objects are also being studied. It is generally believed that gamma rays of energy greater than 1012 eV can be produced only through interactions of high-energy protons or other hadrons. These gamma rays provide important information on the structure and operation of these exotic objects but the key question of whether these particles escape to become galactic cosmic rays remains unanswered.  See also: Astrophysics, high-energy; Binary star; Cerenkov radiation; X-ray astronomy

Direct detection of cosmic rays propagating in distant regions of the Milky Way Galaxy is possible by observing the electromagnetic radiation produced as they interact with other constituents of the Milky Way Galaxy. Measurement of the average electron spectrum using radio telescopes has already been mentioned. Proton intensities are mapped by studying the arrival directions of gamma rays produced as they collide with interstellar gas. Unfortunately, the amount of radiation in these processes depends upon both the cosmic-ray flux and the magnetic-field intensity or density of interstellar gas. Areas where cosmic rays are known to exist can be pointed out because the radiation is observed. But where no radiation is seen, it is not known whether its absence results from lack of cosmic rays or lack of anything for them to interact with. In particular, very little radiation is seen from outside the Milky Way Galaxy, but there is also very little gas or magnetic field there. There is therefore no direct evidence either for or against galactic containment.

A major difficulty with the concept of cosmic radiation filling the universe is the large amount of energy needed to maintain the observed intensity in the face of an expanding universe—probably more energy than is observed to be emitted in all other forms put together.  See also: Cosmology

Confinement mechanisms

Three possible models of cosmic-ray confinement are under investigation. All assume that cosmic rays are produced in sources, discrete or extended, scattered randomly through the galactic disk. Most popular is the “leaky box” model, which proposes that the particles diffuse about in the magnetic field for a few million years until they chance to get close to the edge of the Milky Way Galaxy and escape. This is a phenomenological model in that no mechanism is given by which either the confinement time or the escape probability as a function of energy can be calculated from independent observations of the galactic structure. Its virtue is that good fits to the observed abundances of spallation products are obtained by using only a few adjustable parameters. Variations of the model which mainly postulate boxes within boxes—ranging from little boxes surrounding sources to a giant box or static halo surrounding the whole Milky Way Galaxy—can be used to explain variations from the simple predictions. However, all attempts to calculate the details of the process have failed by many orders of magnitude, predicting ages which are either far older or far younger than the observed age.

A second model is that of the dynamical halo. Like the earlier static-halo model, it is assumed that cosmic rays propagate not only in the galactic disk but also throughout a larger region of space, possibly corresponding to the halo or roughly spherical but sparse distribution of material which typically surrounds a galaxy. This model is based on the observation that the energy density of the material which is supposed to be contained by the galactic magnetic field is comparable to that of the field itself. This can result in an unstable situation in which large quantities of galactic material stream out in a galactic wind similar in some respects to the solar wind. In this case the outward flow is a natural part of the theory, and calculations have predicted reasonable flow rates. In distinction to the solar wind, in which the cosmic rays contribute almost nothing to the total energy density, they may provide the dominant energy source in driving the galactic wind.

A third model assumes that there is almost no escape; that is, cosmic rays disappear by breaking up into protons which then lose energy by repeated collision with other protons. To accept this picture, one must consider the apparent 5-g/cm2 (0.07-lb/in.2) mean path length to be caused by a fortuitous combination of old distant sources and one or two close young ones. Basically, the objections to this model stem from the tendency of scientists to accept a simple theory over a more complex (in the sense of having many free parameters) or specific theory when both explain the data.  See also: Milky Way Galaxy

Acceleration mechanisms

Although the energies attained by cosmic-ray particles are extremely high by laboratory standards, their generation can probably be understood in terms of known astronomical objects and laws of physics. Even on Earth, ordinary thunderstorms generate potentials of millions of volts, which would accelerate particles to respectable cosmic-ray energies (a few megaelectronvolts) if the atmosphere were less dense. Gamma rays and neutrons have, in fact, been detected from lightning strikes. Consequently, there are many theories of how the acceleration could take place, and it is quite possible that more than one type of source exists. Two major classes of theories may be identified—extended-acceleration regions and compact-acceleration regions.

Extended-acceleration regions

Acceleration in extended regions (in fact the Milky Way Galaxy as a whole) was first proposed by E. Fermi, who showed that charged particles could gain energy from repeated deflection by magnetic fields carried by the large clouds of gas which are known to be moving randomly about the Milky Way Galaxy. Many other models based on such statistical acceleration have since been proposed, the most recent of which postulates that particles bounce off shock waves traveling in the interstellar medium. Such shocks, supposed to be generated by supernova explosions, undoubtedly exist to some degree but have an unknown distribution in space and strength, leaving several free parameters which may be adjusted to fit the data.

Compact-acceleration regions

The basic theory in the compact-acceleration class is that particles are accelerated directly in the supernova explosions themselves. One reason for the popularity of this theory is that the energy generated by supernovas is of the same order of magnitude as that required to maintain the cosmic-ray intensity in the leaky box model.

However, present observations indicate that the acceleration could not take place in the initial explosion. Cosmic rays have a composition which is similar to that of ordinary matter and is different from the presumed composition of the matter which is involved in a supernova explosion. At least some mixing with the interstellar medium must take place. Another problem with an explosive origin is an effect which occurs when many fast particles try to move through the interstellar gas in the same direction: the particles interact with the gas through a magnetic field which they generate themselves, dragging the gas along and rapidly losing most of their energy. In more plausible theories of supernova acceleration, the particles are accelerated gradually by energy stored up in the remnant by the explosion or provided by the intense magnetic field of the rapidly rotating neutron star or pulsar which is formed in the explosion.

Acceleration of high-energy particles was first observed in the Crab Nebula, the remnant of a supernova observed by Chinese astronomers in 1054. This nebula is populated by high-energy electrons which radiate a measurable amount of their energy as they spiral about in the magnetic field of the nebula. So much energy is released that the electrons would lose most of their energy in a century if it were not being continuously replenished. Pulses of gamma rays also show that bursts of high-energy particles are being produced by the neutron star—the gamma rays coming out when the particles interact with the atmosphere of the neutron star. Particles of cosmic-ray energy are certainly produced in this object, but it is unlikely that they escape from the trapping magnetic fields in the nebula and join the freely propagating cosmic ray population. As noted above, observation of 1012-eV gamma rays now allows mapping of energetic hadrons in these objects, but the issue of an escape mechanism remains.  See also: Crab Nebula; Neutron star; Pulsar

Acceleration in the solar system

The study of energetic particle acceleration in the solar system is valuable in itself, and can give insight into the processes which produce galactic cosmic rays. Large solar flares, about one a year, produce particles with energies in the gigaelectronvolt range, which can be detected through their secondaries even at the surface of the Earth. It is not known if such high-energy particles are produced at the flare site itself or are accelerated by bouncing off the shock fronts which propagate from the flare site outward through the solar wind. Nuclei and electrons up to 100 MeV are regularly generated in smaller flares. In many events it is possible to measure gamma rays and neutrons produced as these particles interact with the solar atmosphere. X-ray, optical and radio mappings of these flares are also used to study the details of the acceleration process. By relating the arrival times and energies of these particles at detectors throughout the solar system to the observations of their production, the structure of the solar and interplanetary magnetic fields may be studied in detail.

In addition to the Sun, acceleration of charged particles has been observed in the vicinity of the Earth, Mercury, Jupiter, and Saturn—those planets which have significant magnetic fields. Details of the acceleration mechanism are not understood, but certainly involve both the rotation of the magnetic fields and their interactions with the solar wind. Jupiter is such an intense source of electrons below 30 MeV that it dominates other sources at the Earth when the two planets lie along the same interplanetary magnetic field line of force. The anomalous component is known to be local interstellar material, accelerated by a mechanism that is not fully understood but probably is situated near the interface between the solar wind and the interstellar medium. However, there was no evidence in the Voyager 1 crossing of the termination shock that this production occurs immediately at this particular structure.  See also: Jupiter; Mercury (planet); Planet; Saturn

Direct observation of conditions throughout most of the solar system will be possible in the next few decades, and with it should come a basic understanding of the production and propagation of energetic particles locally. This understanding will perhaps form the basis of a solution to the problem of galactic cosmic rays, which will remain for a very long time the main direct sample of material from the objects of the universe outside the solar system.

Table 1: Mean free paths for primary cosmic rays in the atmosphere
Charge of primary nucleus	Mean free path in air, g/cm2*
Z = 1	60
Z = 2	44
3 ≤ Z ≤ 5	32
6 ≤ Z ≤ 9	27
10 ≤ Z ≤ 29	21