Astronomy and Astrophysics for the 1980's, Volume 2 Reports of the Panels (1983) / Chapter Skim
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VII. Cosmic-Ray Astronomy
VII. Cosmic-Ray Astronomy
Pages 55-68
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From page 55... ...
On the one hand, they carry in the details of their composition and energy spectra interesting and unique information about their sources and the regions of space in which they have traveled; on the other hand, they are an important astrophysical entity in themselves, having an energy density comparable with that of the Galactic magnetic field and of the turbulent motion of the interstellar gas. The pressure of the cosmic-ray gas and its heating of the interstellar medium affect the processes of star formation and influence the structure and evolution of the Galaxy.
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From page 56... ...
Large and heavy instruments have been used at high altitudes in balloon flights and in Earth orbit on the HEAD-3 satellite to measure the composition and spectra of cosmic rays up to energies of 101~ eV. Very large arrays of detectors on the ground have been used to study cosmic rays with energies as high as 1012 eV through the observation of showers of secondary particles produced in the atmosphere It is believed that most cosmic rays originate in processes associated with supernovae and are confined within the Galaxy for millions of years by the Galactic magnetic field.
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From page 57... ...
The average thickness, or pathlength, of the matter traversed before escape from the Galaxy is determined from measurements of the relative abundances of the secondary cosmic rays produced in these interactions, such as the nuclei produced by fragmentation of heavier nuclei in collisions with interstellar matter. If one also determines the average containment time, for instance through measurements of the abundances of various radioactive isotopes,
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From page 58... ...
The importance of this effect depends critically on the flux of Galactic cosmic rays at very low energy. This flux is still unknown, because lowenergy cosmic rays are swept out of the solar system by the solar magnetic field moving outward with the solar wind.
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From page 59... ...
Instruments on solar satellites and deep space probes, which include Pioneer-10 and -11, Mariner-10, Helios-1 and -2 and Voyager-1 and -2, measured the composition and spectra of cosmic rays over a wide range of heliocentric distances and surveyed the particle populations and acceleration phenomena in the magnetospheres of Jupiter and Saturn. Skylab, at the beginning of the decade, carried plastic track detectors in Earth orbit to measure the composition of the rare ultraheavy nuclei.
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From page 60... ...
An "anomalous component" of cosmic rays with very low energies of about 10 MeV/nucleon or less has been discovered. Its most likely origin is neutral atoms of interstellar matter, photoionized in the neighborhood of the Sun and accelerated by magnetohydrodynamic turbulence in the solar cavity.
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From page 61... ...
In the region of lower nuclear charges (Z up to 40) the elemental composition is clearly not dominated by the products of reprocess nucleosynthesis, and the difference between cosmic-ray and solar-system abundances are roughly correlated with the values of the first ionization potentials.
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From page 62... ...
Particularly important are the abundances of the neutronrich isotopes of neon, magnesium, silicon, sulfur, iron, and nickel, all of which are sensitive to circumstances of the initial phase of a supernova explosion. Information about the Galactic containment time of cosmic rays can be derived from measurements of the relative abundances of radioactive isotopes.
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From page 63... ...
As noted above, the average pathlength of cosmic-ray nuclei at these high energies is probably less than 1 g/cm2. Thus abundance changes due to interstellar spallation are almost negligible, and the measurements will yield direct information on the elemental composition at the acceleration site.
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From page 64... ...
Better knowledge of shower development and accurate calibrations of detectors are needed to improve the reliability with which the energy and composition of the ultra-high-energy primaries are deduced from observations of showers. Years of exposure time with detectors of the largest attainable effective collecting areas will be required in order to obtain adequate statistical accuracy in the measurement of the spectrum and the distribution in arrival directions.
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From page 65... ...
Small Satellites and Space Probes Several spacecraft or deep-space probes with small cosmic-ray detectors aboard are expected to remain active into the 1980's. Detectors on ISEE-1 and -3 will continue to measure the low-energy elemental composition and low-energy electrons in interplanetary space and provide the isotopic composition of the more abundant nuclides.
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From page 66... ...
Of central importance to the field in the 1980's are long exposures of large instruments in near-Earth orbit and high-sensitivity isotopic composition measurements on spacecraft beyond the interference of the Earth's magnetosphere. Experiments on currently active satellites and space probes should be fully utilized, and future planetary missions should be equipped with appropriate particle detectors.
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From page 67... ...
Other scientific objectives include measurement of the elemental composition at low energies, detailed studies of the anomalous component, and investigations of particles of interplanetary origin. Simultaneous measurements at different locations in the heliosphere and over a long period of time are necessary.
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From page 68... ...
Opportunities for deep-space observations on future outer planetary missions should be utilized in order to enhance the probability that a properly functioning spacecraft will eventually leave the region of solar modulation, even though the time required for the journey significantly exceeds nominal mission and spacecraft design lifetimes.
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