Opportunities in Cosmic-Ray Physics and Astrophysics

Studies of energetic particles from distant regions of the galaxy and the universe bring us information about the processes in which the particles are accelerated to relativistic energies, about the role of the particles and their accelerators in driving dynamical processes in our galaxy and beyond, and about the distribution of matter and fields in interstellar space. This information is complementary to astronomy with photons in various wavelength bands.

The field of cosmic-ray physics has evolved sufficiently so that the general outlines of a theory of the origin of cosmic rays are visible, but as the field has evolved, important questions have been raised. What is the physical origin of the similarities between galactic cosmic rays and solar flare particles? What is the maximum energy to which supernova blast waves can accelerate particles? Can we find specific point sources of high-energy cosmic rays? Is there a high-energy component of protons or nuclei from distant, extragalactic sources, and how does it interact with microwave background radiation on cosmological time scales? What new sources may be responsible for the few highest-energy particles observed recently with the largest ground-based detectors? Are there signals among cosmic-ray positrons or antiprotons of the existence of dark matter? Are there cosmic-ray antinuclei?

There is much new activity aimed at answering these questions, including efforts by scientists previously working in other fields who have been attracted by the scientific interest of some of the problems. The committee's recommendations reflect this new activity and interest. Cosmic-ray physics is an interdisciplinary field, both in the nature of the scientific problems it addresses and in the techniques it uses. Thus, this report emphasizes the need for the granting agencies to be aware of and responsive to initiatives that sometimes cross the boundaries of traditional scientific disciplines.

Summary Recommendations

• NASA should provide the opportunity to measure cosmic-ray electrons, positrons, ultraheavy nuclei, isotopes, and antiparticles in space.

These measurements are needed to identify the sources of the material that gets accelerated; to understand the time scales for injection, acceleration, and propagation; and to search for possible exotic sources of cosmic rays. They will also lead to greater understanding of the structure and dynamics of the plasma surrounding the Sun. Small, low-cost missions, such as those carried out in NASA's Explorer program, can address each of these issues. Because of the long lead time needed for space experiments, it is essential that there be strong support for balloon payloads. In some cases, significant scientific results can be achieved with such suborbital exposures; in addition, this activity is of great benefit in the development of future payloads for space and in training students to work in a variety of fields that require advanced technical skills.

• NASA, NSF, and DOE should facilitate direct and indirect measurement of the elemental composition to as high an energy as possible. Support of long-duration ballooning and support of hybrid ground arrays will be needed to accomplish this end.

The goal here is to look for a maximum energy associated with acceleration by supernova-driven shocks, which is expected to occur around 10 eV. Since the cosmic-ray spectrum continues to higher energy, a cutoff in one type of source would imply a transition to a new source capable of accelerating particles to higher energy. Understanding whether and how such a transition occurs is an important objective. There is already an indication for some structure in the energy spectrum near 10 eV. Because of the low intensity of cosmic rays at such high energy, however, present direct measurements with detectors flown above the atmosphere have not accumulated enough data to clarify its significance. Long-duration balloon flights of detectors will extend direct measurements by an order of magnitude in energy. Hybrid ground arrays, overlapping with direct measurements at the low-energy end of their range, will extend the study of composition beyond the "knee" of the spectrum to much higher energy.

• NSF and DOE should support the new Fly's Eye and provide for U.S. participation in the big projects on the horizon, which include giant arrays, ground-based -ray astronomy, and neutrino telescopes.

In the energy range above 10 eV there is possible evidence from large air-shower experiments for a transition to a different, high-energy component of the cosmic radiation. This component may originate far outside our galaxy, or at least from its outermost reaches. The High-Resolution Fly's Eye will be able to explore this region with unprecedented energy resolution. Giant arrays spread over thousands of square kilometers are needed to accumulate sufficient statistics to study the very highest-energy particles, which probe cosmological distances and may lead to the discovery of new, energetic astrophysical sources.

The main thrust of ground-based g-ray astronomy is to extend present measurements to lower energy and to greater levels of sensitivity in order to study the variety of galactic and extragalactic sources that are being discovered at lower energy with detectors on the Compton Gamma-Ray Observatory. Objects such as active galactic nuclei may be high-energy particle accelerators producing secondary photons and neutrinos at the source. It is desirable to extend measurements of spectra of g rays from these and other point sources beyond the energies of present space experiments to understand these sources better. If sufficiently large neutrino telescopes can be built, a comparison between neutrino and photon fluxes from the same objects or classes of objects could be made that will be very helpful in understanding physical processes within the source. Because of their great penetrating power, neutrinos probe activity deep within sources where the corresponding photons would be reabsorbed.

• NASA, NSF, and DOE should support a strong program of relevant theoretical investigations.

Examples of key problems are sources and propagation of the highest-energy particles; how the details of the source and the magnetic field geometry determine the maximum energy of a particular accelerator; and the relation between acceleration of electrons and acceleration of ions.

These recommendations cover a broad range of topics and techniques, yet the underlying astrophysical processes that we seek to understand, taken together, form a coherent whole. One unifying theme is particle acceleration on a variety of scales; another is the role of energetic particles in the dynamics of the universe, again on many scales, from the heliosphere to supernova remnants, to the galactic disk and halo, and beyond to clusters of galaxies and distant active galaxies. Ultimately, cosmic rays are tracers of the processes by which the elements, synthesized in stars, are dispersed and reprocessed by energetic processes such as stellar winds, supernova explosions, and jets driven by accretion onto compact objects. The committee concludes that carrying out the recommendations of this report will lead to significant advances in our understanding of these processes from which important new insights and discoveries are likely.

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