Cosmic-ray lifetime in the galaxy: Experimental results and models

The containment lifetime of the cosmic radiation is a crucial parameter in the investigation of the cosmic-ray origin and plays an important role in the dynamics of the Galaxy. The separation of the cosmic-ray Be isotopes achieved by two satellite experiments is considered in this paper, and from the measured isotopic ratio between the radioactive ¹⁰Be (half-life = 1.5 × 10⁶ yr) and the stable ⁹Be, it is deduced that the cosmic rays propagate through matter with an average density of 0.24 ± 0.07 atoms cm^-3, lower than the traditionally quoted average density in the galactic disk of 1 atom cm^-3. This paper reviews the implications of this result for the cosmic-ray age mainly in the context of two models of confinement and propagation: the homogeneous model, normally identified with confinement to the galactic gaseous disk, and a diffusion model in which the cosmic rays extend into a galactic halo. The propagation calculations use:

1. a newly deduced cosmic-ray pathlength distribution.
2. a self-consistent model of solar modulation.
3. an up-to-date set of fragmentation cross sections.

The satellite results and their implications are compared with the information on the cosmic-ray age derived from other cosmic-ray radioactive nuclei and the measured differential energy spectrum of high-energy electrons. It is a major conclusion of this paper that in a homogeneous model the cosmic-ray age is 15(+7, -4) million years, i.e., about a factor 4 longer than early estimates based on the abundances of the light nuclei Li, Be, and B and a nominal interstellar density of 1 atom cm ^-3. The lifetime is even longer when the satellite results are applied to a diffusion halo model. The deduced traversed matter density, together with other astrophysical considerations, suggest the population of a galactic halo by the cosmic rays.     

The Origin of Primary Cosmic Rays: Constraints from ACE Elemental and Isotopic Composition Observations

Cosmic-ray isotope observations from NASAs Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to ⁵⁶Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes ⁴⁸Ca and ⁶⁴Ni. Although the source abundances range over a factor 10⁴, most of the ratios to ⁵⁶Fe are consistent with solar-system values to within 20%. However, there are some notable differences, the most significant being an excess of (70±30)% relative to the solar system for the cosmic-ray source ratio ⁵⁸Fe/⁵⁶Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.     

Propagation, Confinement Models, and Large-Scale Dynamical Effects of Galactic Cosmic Rays

The problems of cosmic-ray transport in the Galaxy are discussed. The discussion covers the diffusion model of cosmic ray propagation in the Galaxy, the dynamical effects of relativistic particles in the interstellar medium, and the origin of ultra high energy cosmic rays.     

Three-dimensional cosmic-ray simulations: Heliographic latitude and current-sheet tilt

We present the results from a study of the variations of the cosmic-ray intensity with time, heliographic latitude, and longitude, and for varying interplanetary conditions, using our three-dimensional, time-dependent computer code for cosmic-ray transport in the heliosphere. Our code also produces a solar-wind and interplanetary magnetic field (IMF) configuration which is compared with observations. Because of the fully threedimensional nature of the model calculations, we are able to model time variations which would be expected to be observed along Ulysses's trajectory as it moves to high latitudes. In particular we can model the approximately 13-and 26-day solar-rotation induced variations in cosmic rays, solar wind and IMF, as a function of increasing heliographic latitude, as one moves poleward of the interplanetary current sheet. Our preliminary model results seem to be in general form quite similar to published data, but depend on the physical parameters used such as cosmic-ray diffusion coefficients, boundary conditions, and the nature of the solar wind and IMF and current sheet.     

The theory of cosmic-ray modulation

The current state of the theory describing cosmic-ray modulation in the interplanetary medium is reviewed. Emphasis is given to the problems of determining the transport coefficient for diffusion in energy and position space and in assessing the importance of particle drift motion in three-dimensional modulation models.     

Modeling of 3-D Corotating Cosmic-Ray Structures in the Heliosphere

We present a brief review of the modeling of corotating 3-dimensional features in heliospheric cosmic rays. The model heliosphere incorporates a wavy current sheet and Corotating Interaction Regions (CIRs). We find that present models can qualitatively account for the observed extension of recurrent 26-day cosmic-ray variations to high heliospheric latitudes if perpendicular diffusion is significant. The recurrent enhancement of low-energy (MeV) particles accelerated at CIR-s is also shown to fit into this same picture.     

The origin of cosmic rays in spiral galaxies

The problem of the origin and distribution of cosmic rays in the Galaxy is introduced by summarizing the literature on the radio and -ray studies of the Galaxy, discussing the propagation of cosmic rays in the interstellar medium, and listing the observed properties of cosmic rays. The localization of cosmic-ray electrons to their parent galaxies is an indicator that processes leading to cosmic-ray production may be common to galaxies like our own. The studies of external galaxies are therefore relevant to our own and have the advantage of better perspective.Studies of cosmic rays in exsternal galaxies are limited to the electron component which radiates synchrotron emission at radio frequencies. Multi-colour photometry of galaxies allows the separation of stellar populations that harbour particular classes of cosmic-ray sources. Statistical studies aimed at correlating integrated radio and optical properties of galaxies have reached conflicting conclusions. Although a correlation of cosmic rays with the older stellar population is proposed by some authors, others argue that the young stellar population harbours cosmic ray sources.Morphological studies of resolved galaxies provide information on the distributions of cosmic-ray electrons in galaxies. Studies in which the resolution of the radio images is much lower than in the optical are limited and have also produced contradictory results. Radio imaging at optical resolution is required for a direct comparison of cosmic-ray distributions with stellar distributions. Such studies are reviewed and the constraints they impose on cosmic-ray propagation and distribution of cosmic-ray sources is discussed.Theoretical cosmic-ray acceleration mechanisms are surveyed and an attempt is made to determine likely contributors. Mechanisms associated with shock waves in a variety of astrophysical settings are reviewed. Acceleration mechanisms not involving shocks, are also discussed. Finally, the status of the field is summarized along with some speculation on the future directions the field may take.     

Neutron Monitor Records in Broader Historical Context

Man-made neutron monitors have provided a continuous detailed record of the cosmic-ray flux over only about the last 5 decades. Fortunately, nature operates its own detectors and offers the opportunity to extend the cosmic-ray records over much longer time scales. Two different types of `natural detectors' can be distinguished. The first is based on long lived radionuclides that are produced by cosmic-ray interactions in the atmosphere and subsequently become stored in archives such as ice sheets or tree rings. The second type are rocks that are exposed to cosmic-rays at a certain time and from then on integrate the production of cosmogenic nuclides over the whole exposure time. The analysis of ¹⁰Be in polar ice cores and ¹⁴C in tree rings clearly reveals solar and geomagnetic modulation of the cosmic-ray flux on different time scales ranging from decades (11-year Schwabe cycle) to millennia. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cosmic-Ray Transport and Interaction with Shocks

This paper reviews selected topics in cosmic-ray transport in the heliosphere, as well as recent insights on the interaction of cosmic rays with shocks. Topics include: (a) recent observations suggesting very long inferred scattering mean-free paths of cosmic rays, (b) recent insights into the diffusion of cosmic rays normal to a magnetic field, (c) the physics of super-diffusion and sub-diffusion, and (e) the interaction of cosmic rays with shocks moving through large-scale irregular magnetic fields.     

The Astrophysics of Galactic Cosmic Rays

Observations of cosmic rays and their related radio to gamma-ray signatures are surveyed and discussed critically, and compared to theoretical models of the cosmic-ray origin and propagation. The analogous heliospheric processes are included as a well-studied case of the principal physical processes of energetic particle acceleration and propagation. Reinforcements, or conflicts, in the interpretations of cosmic-ray spectral and compositional characteristics arise when cosmic-ray source and propagation models are confronted with astronomical information about the Galaxy as a whole and from potential source sites, i.e., supernova remnants or regions with high massive-star density. This volume represents the outcome of two workshops held at ISSI. In this chapter we summarize the introductory papers presented below, and include insights from the workshop discussions.     

On The Stratospheric Measurements of Cosmic Rays

The results of measurements of cosmic-ray intensity in the Earth's atmosphere from 1957 up to now are presented. Balloon launchings performed several times a week at northern and southern polar latitudes as well as at midlatitudes yield a homogeneous data series for the study of the spatial distribution and temporal changes in the cosmic-ray intensity. The 11- and 22-year solar modulation, north-south asymmetry, energy hysteresis and a long-term trend in the cosmic-ray intensity are discussed. The fluxes and energy spectra of protons with energy > 100 MeV for about 100 solar energetic particle events in 1958-1992 are given. The role of cosmic rays in terrestrial atmospheric processes is briefly discussed.     

What are the Limits for ace Galactic Cosmic-ray Isotope Studies?

The CRIS experiment on ACE, with its excellent charge and mass resolution and a geometrical factor ∼10× that of any previous experiment, holds the promise of rewriting the book on galactic cosmic-ray abundance studies. Translating these measurements into precise cosmic-ray source abundances and using these measurements to determine more accurately the propagation history of cosmic rays is a different matter, however. In many important cases these studies will be limited by the accuracy of the nuclear cross- sections that determine how the cosmic-ray composition is modified as it traverses the interstellar matter. In this paper we will discuss these cross-sections and how well they are known as a function of the energy and the charge and mass of the cosmic-ray nuclei. This will then be used to discuss what new limits can be expected on several contemporary problems of interest in cosmic rays from the CRIS measurements. This revised version was published online in June 2006 with corrections to the Cover Date.     

KEY MEASUREMENTS IN THE FUTURE – Working Group Report

The experimental basis of cosmic-ray astrophysics consists of detailed measurements of the cosmic-ray intensity arriving near earth, of observations of photons in all wavelength bands generated by cosmic ray interactions in the interstellar medium or in the cosmic-ray sources, and of laboratory studies of high energy particle interactions. In addition, a large body of astronomical information on the composition of stellar atmospheres and of the interstellar medium, including interstellar dust grains, is required to bring cosmic-ray data into context with subjects such as nucleosynthesis and evolution of the galaxy. This report will summarize some of these observational questions, will discuss specific experimental needs in current research, and will review some of the key measurements that can be expected for the near future. This review will neither be complete nor attempt to establish observational priorities. However, it will illustrate the variety of observational activities that are required to achieve progress.     

Heavy Elements and Age Determinations

The age of the universe, measured from the Big Bang to the present, is at the focus of cosmology. Its determination relies, however, on the use of stellar objects or their products. Stellar explosions, like type Ia supernovae serve as standard(izable) candles to measure the expansion of the universe. Hertzsprung––Russell diagrams of globular clusters can determine the age of such clusters and thus are lower limits of the age of the galaxy and therefore also the universe. Some nuclear isotopes with half–lives comparable to the age of galaxies (and the universe) can serve as clocks (chronometers) for the duration of nucleosynthesis. The isotopes ²³⁸U and ²³²Th with half–lives of 4.5×10⁹ and 1.4×10¹⁰ yr, decaying via alpha decay chains to Pb isotopes, are well suited to serve this purpose. They are products of the same nucleosynthesis process, the r-process. Therefore, the present paper aims at understanding the necessary environment conditions in the (stellar) production sites, the nuclear physics involved, the observational constraints for r-process nucleosynthesis, the results from nucleocosmochronology, and the remaining challenges and uncertainties which need to be overcome for a full understanding of the nature of the r-process.     

Analyses of sidereal and solar anisotropies in cosmic rays

The Sun's interplanetary magnetic field and the solar wind modulate the distribution of galactic cosmic-ray particles in the heliosphere. The particles diffuse inward, convert outward and have drifts in the motion of their gyro-centres. Irregularities in the IMF also scatter particles from their gyro-orbits. These processes are the components of solar modulation and produce streaming (and higher-order anisotropies) of particles in the heliosphere. The anisotropies can be investigated at the Earth by examining the count rates of cosmic-ray detectors. The anisotropic streams appear as diurnal variations in solar and sidereal time in the count rates. Higher-order anisotropies produce generally much smaller semi-diurnal and higher-order variations. Theoretical models of solar modulation predict effects that depend on the polarity of the Sun's magnetic dipole. The solar diurnal and north-south anisotropies can be used to test these predictions. This paper is a short review of analyses of 60 years of cosmic-ray data collected at the Earth for the solar and sidereal diurnal variations present. Past analyses have yielded interesting and controversial results regarding the rigidity spectra and components of these anisotropies. Some of the controversy remains today. Analyses of these anisotropies have also yielded quantitative information about parameters important to solar modulation, such as latitudinal and radial density gradients. The relatively new techniques used for these determinations are explained here. Calculations of these modulation parameters from Earth-based cosmic-ray detectors are reviewed and compared to spaceprobe measurements and theoretical predictions of their values. Recently, investigations of the sidereal and solar diurnal anisotropies have been combined to calculate mean-free-paths of cosmic rays in the heliosphere. The latest conclusions from these analyses are that the parallel mean-free-paths of cosmic rays may depend on the polarity of the Sun's magnetic field. The results of these investigations are included in this paper to indicate the present state of knowledge concerning this facet of cosmic-ray research.Now at Department of Physics, Shinshu University, 3-1-1 Asahi, Matsumoto 390, Japan.     

Upstream waves and particles

A brief analysis is made of the interrelation of the intensity of cosmic-ray particles, the column density of gas and the intensity of cosmic γ-rays. It is shown that, locally, γ-ray data enable the calibration of H₂ densities to be inferred from CO data and elsewhere the variation of cosmic-ray intensity with position to be assessed. Finally, the importance of cosmic-ray irradiated molecular clouds in simulating γ-ray ‘sources’ is reiterated.     

Galactic effects of the cosmic-ray gas

On an astronomical scale cosmic rays must be considered a tenuous and extremely hot (relativistic) gas. The pressure of the cosmic-ray gas is comparable to the other gas and field pressures in interstellar space, so that the cosmic-ray pressure must be taken into account in treating the dynamical properties of the gaseous disk of the galaxy. This review begins with a survey of present knowledge of the cosmic-ray gas. Then the kinetic properties of the gas are developed, followed by an exposition of the dynamical effects of the cosmic-ray gas on a large-scale magnetic field embedded in a thermal gas. The propagation of low-frequency hydromagnetic waves is worked out in the fluid approximation.The dynamical properties of the gaseous disk of the galaxy are next considered. The equations for the equilibrium distribution in the direction perpendicular to the disk are worked out. It is shown that a self-consistent equilibrium can be constructed within the range of the observational estimates of the gas density, scale height, turbulent velocity, field strength, cosmic-ray pressure, and galactic gravitational acceleration. Perturbation calculations then show that the equilibrium is unstable, on scales of a few hundred pc and in times of the order 2 × 10⁷ years. The instability is driven about equally by the magnetic field and the cosmic-ray gas and dominates self-gravitation. Hence the instability dominates the dynamics of the interstellar gas and is the major effect in forming interstellar gas clouds. Star formation is the end result of condensation of the interstellar gas into clouds, indicating, then, that cosmic rays play a major role in initiating star formation in the galaxy.The cosmic rays are trapped in the unstable gaseous disk and escape from the disk only in so far as their pressure is able to inflate the magnetic field of the disk. The observed scale height of the galactic disk, the short life (10⁶ years) of cosmic-ray particles in the disk of the galaxy, and their observed quiescent state in the disk, indicate that the galactic magnetic field acts as a safety valve on the cosmic ray pressure P so that PB ²/8. We infer from the observed life and quiescence of the cosmic rays that the mean field strength in the disk of the galaxy is 3–5 × 10^–6 gauss.     

The Solar Isotope Spectrometer for the Advanced Composition Explorer

The Solar Isotope Spectrometer (SIS), one of nine instruments on the Advanced Composition Explorer (ACE), is designed to provide high- resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z=2 to 30) over the energy range from ∼10 to ∼100 MeV nucl−1. During large solar events SIS will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet times SIS will measure the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic-ray component, which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories are measured with a pair of two-dimensional silicon-strip detectors instrumented with custom, very large-scale integrated (VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry factor of ∼40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. A microprocessor controls the instrument operation, sorts events into prioritized buffers on the basis of their charge, range, angle of incidence, and quality of trajectory determination, and formats data for readout by the spacecraft. This paper describes the design and operation of SIS and the scientific objectives that the instrument will address. This revised version was published online in June 2006 with corrections to the Cover Date.     

Some problems and perspectives in cosmic-ray studies

The principles of an ionisation calorimeter, an instrument used to measure the energy of cosmic-ray particles and the dependence of its parameters on the conditions of operation, are discussed.Possible applications of this calorimeter in the study of nuclear interactions of 10¹¹–10¹³ eV cosmic-ray particles, study of the composition of high-energy primary cosmic rays (10¹¹–10¹⁴eV), and investigation of the electron component of cosmic-ray primaries and high-energy -rays are reviewed.Translated by Express Translation Service, Wimbledon, London.