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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
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 exceptionalonly 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
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
1 18 COSMIC RA YS 10' 10° 1 0-] ° 10-2 lo \ 1 0-3 > a) in 10 4 an ~ 10-5 x lo 6 ~ 10 7 1 o-8 1 0-9 - _ ~ Can ~ ~ AWAY Fin ~ ~ H 1~, ~ 't ,, ,,,,,,1 ,,, 1 01 ~ it\ ,,,,, 1 , , ,,,, 1 ._ 102 103 104 105 1o6 107 (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.
OVERVIEW 1 19 N-N C.M. ENERGY i/ , (TeV) 0.1 1.0 10 100 I! 10 ~ 10 -10 10 Lo he -12 lo-14 · . . : \ : z Lo c~ ~ ; ~ :~ Q :> 1~ . O) ~ . - 10 10 lol4 lol5 lol6 1017 1018 lol9 1020 (B) ENERGY (eV) lo4 ~ LLI 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
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.
