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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
| Table 2: Properties of particles when all have a rigidity of 1 gigavolt | |||||||||||
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- V. S. Berezinskii et al., Astrophysics of Cosmic Rays, 1991
- R. Clay and B. Dawson, Cosmic Bullets, 1997
- M. W. Friedlander, Cosmic Rays, 1989
- T. K. Gaisser, Cosmic Rays and Particle Physics, 1990
- P. K. F. Greider, Cosmic Rays at Earth: Researcher's Reference Manual and Data Book, 2001
- M. B. Kallenrode, Space Physics: An Introduction to Plasmas and Particles in the Heliosphere and Magnetospheres, 2d ed., 2001
- M. S. Longair, High Energy Astrophysics, vol. 1: Particles, Photons, and Their Detection, 2d ed., 1992
- J. M. Matthews, High Energy Astrophysics: Theory and Observations from MeV to TeV, 1994
- L. I. Miroshnichenko, Solar Cosmic Rays, 2001
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