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OCR for page 115
15
Overview
Because cosmic rays give us a direct sample of matter from some of
the most energetic processes in nature and from distant regions of
space, interest in the field remains high despite the difficulty of
associating the particles with individual sources. Indeed, unraveling
the physics of the acceleration of cosmic rays and of their propagation
in the turbulent interstellar medium in order to discover the nature of
the sources is a principal activity of the field.
The material of the solar system represents the local interstellar
material as it was 4.6 billion years ago. The much younger cosmic-ray
material, accelerated about 10 million years ago, provides a different
sample of matter. In fact, recent observations suggest that cosmic rays
may actually represent a more typical sample of the average interstellar
medium than the solar-system material, which may have been contam-
inated by a nearby supernova explosion. The differences between
cosmic rays and solar-system material are both significant and subtle.
Understanding them will require more and better experimental data,
perhaps new scenarios of nucleosynthetic processes, and a better un-
derstanding of the acceleration processes for the cosmic rays. In this
way measurements of isotopic and elemental abundances will make
important contributions to studies of the origin of the elements, a field
that has only a limited amount of real data with which to check its
theories.
There is much still to be learned about the nature of the material we
115
OCR for page 116
1 16 COSMIC RA YS
observe at Earth as cosmic rays. Some elements (Ne, Mg, and Si) have
been observed to have unexpected isotopic composition; models of
cosmic-ray origin that could explain these compositions have been
proposed, and observations of the isotopic composition of other ele-
ments are required to distinguish among these models. Studies of the
abundances of the heaviest elements platinum, lead, thorium, and
uranium are still primitive; much better observations are required if
we are to determine the site and time scale of cosmic-ray nucleosyn-
thesis. Observations of electron and positron spectra at higher energies
and with greater precision are required if we are to determine the
distribution of cosmic-ray acceleration sites in the local parts of the
galaxy. Recent measurements of antiprotons at least require significant
modification of simple models of cosmic-ray confinement in the galaxy
and could also indicate more exotic sources; extension of antiproton
observations to higher energies are required to distinguish among these
possibilities. No antinuclei heavier than antiprotons have yet been
observed in the cosmic rays, but if these searches could be extended at
least two orders of magnitude in sensitivity, there is reason to believe
that they would begin to be sensitive to extragalactic matter where
these searches would take on much greater significance.
Nature demonstrates in many places its ability to accelerate parti-
cles. Solar energetic particles are accelerated at the Sun, particles are
accelerated by the magnetospheres of the Earth and Jupiter, and under
certain conditions particles are also accelerated in the interplanetary
medium. The scale for acceleration of galactic cosmic rays is much
larger, and far greater amounts of energy are involved. We see evi-
dence for particle acceleration on an even larger and more energetic
scale when we look at quasars and radio galaxies. Recently the binary
object Cygnus X-3 has been observed with its characteristic 4.8-hour
period by ground-based air-shower arrays in 10~5-eV gamma rays. If, as
is likely, these are secondaries of nuclear collisions, this is good
evidence of an energetic, distant source of cosmic rays in our galaxy.
This could also imply a source of detectable neutrinos. Particle
acceleration is evidently a common occurrence in a wide variety of
astrophysical settings.
The total energy required to keep the galaxy filled with cosmic rays
is enormous; it requires a substantial fraction of the energy released by
massive stars such as supernova exploding at the rate of one every 30
years somewhere in the galaxy. The energy given to each cosmic-ray
particle is also enormous; cosmic rays are truly exceptional—only one
particle in 10'~ in our galaxy becomes a cosmic ray, the most common
cosmic rays have 10~° times as much energy as the thermal energy of
OCR for page 117
OVERVIEW 117
a typical atom on Earth, and the most energetic cosmic rays are 10~
times still more energetic. Thus, cosmic rays play a crucial role in the
energy balance of the interstellar medium.
Recent theoretical developments involving acceleration in various
kinds of astrophysical shocks begin to make possible an understanding
of the acceleration processes and, for the first time, lead to predictions.
Measurements over the next decade should be able to test these
theories through improved observations of the cosmic-ray energy
spectra.
In their 10-million-year lifetime, the bulk of the cosmic-ray particles
spiral around magnetic field lines, diffuse through the galaxy, and
experience both nuclear and electromagnetic forces within a confine-
ment volume whose size is still uncertain. Experimental data now put
significant constraints on the details of the propagation and the
conditions in the confinement region. The cosmic rays themselves also
affect conditions in their confinement volume by ionizing material in
molecular clouds, ''blowing out" magnetic field lines, and generating
secondary particles and photons through several different nuclear and
electromagnetic processes. Major components of the diffuse radio and
gamma-ray backgrounds are produced by cosmic rays. It is this
intimate relation between the cosmic particle radiation and a broad
range of physical processes that makes cosmic-ray studies such an
important astrophysical discipline.
At some energy around 10~4-10~5 eV or above, galactic acceleration
and containment mechanisms must begin to fail. Nevertheless, the
measured spectrum of cosmic rays extends to around 102° eV without
any sign of a termination. (See Figure 15. 1.) Anisotropy of the cosmic
rays increases continuously from a few tenths of a percent in amplitude
around 10'5 eV to more than 10 percent around 10~9 eV. One recent
analysis suggests that this is consistent with the increased difficulty of
containing galactic cosmic rays and that extragalactic cosmic rays
predominate only above 10'9 eV. At the highest observed energies
(about 109° eV) it appears that cosmic-ray protons would be too
energetic to be trapped in the known magnetic field of our galaxy or to
survive energy loss by photoproduction on the relic blackbody radia-
tion in propagation over cosmological distances. Cosmic rays of such
high energies might come to us from our own local supercluster of
galaxies, or they might come from the core of our own galaxy, bent
back to the galactic plane by the (unknown) magnetic fields in a galactic
halo. In any case these ultrahigh-energy cosmic rays are uniquely
interesting and significant probes of cosmology and astrophysics.
The field of cosmic rays above 10'5 eV forms a bridge between
OCR for page 118
1 18 COSMIC RA YS
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(A) Kinetic Energy (MeV/Nucleon)
FIGURE 15.1 (A) The energy spectra of the cosmic rays measured at Earth. Differ-
ential energy spectra for the elements (from top) hydrogen, helium, carbon, and iron.
The solid curve shows the hydrogen spectrum extrapolated to interstellar space by
unfolding the effects of solar modulation. The turn-up of the helium flux below ~60 MeV
n-' is due to the additional flux of the anomalous 4He component. From J. A. Simpson,
Annul Rev. Nucl. Particle Sci. 33, 323 (1983). (B) The high-energy portion of the
cosmic-ray spectrum (integral). Above 103 eV the composition is not yet well deter-
mined. The vertical bars indicate equivalent laboratory energies of existing and proposed
(SSC) colliding-beam facilities.
OCR for page 119
OVERVIEW 1 19
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high-energy particle physics and experimental astrophysics. At and
above these energies (the highest reached by the present generation of
hadron colliding beams), the energy spectrum and chemical composi-
tion are accessible only by observations of cascades in the atmosphere
with ground-based detectors.
Because the flux of the primary cosmic rays is so low at these
energies, the relatively small detectors in spacecraft or balloons cannot
intercept a large enough number for study. Large detectors can be
OCR for page 120
120 COSMIC RAYS
exposed for periods of years on the ground to overcome this problem,
but then the primary cosmic rays can only be seen indirectly through
the shield of the atmosphere, which is some 10-15 interaction lengths
thick. Ground-based detectors observe extensive air showers the
cascades of particles created by interactions of the primary cosmic rays
high in the atmosphere. Because the energy is so high, the nature of
strong interactions at higher energies (which determines how the
cascades develop) must be inferred from extrapolations from acceler-
ator data and from the indirect cosmic-ray data themselves. Because
the interpretation of cosmic-ray cascades in terms of particle physics
depends on the identity of the initiating cosmic ray (e.g., proton,
carbon, or iron nucleus) and vice versa, our understanding of both
areas is interrelated, and progress is made in an iterative, bootstrap
manner as we move to higher energies. With the prospect of longer
exposures in space we can expect the boundary between direct and
indirect measurements to approach 10~5 eV, and this will help to clarify
the interpretation of the ground-based cascade studies at higher
energies as well.
Representative terms from entire chapter:
interstellar medium
