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.     

Cosmic rays in the outer solar system

A space mission to Jupiter and Saturn, and beyond, provides an opportunity to explore the low energy galactic cosmic rays, which are largely excluded from the inner solar system by the outward sweep of the magnetic fields in the solar wind. The low energy cosmic rays are believed to be responsible for much of the heating of the gaseous disk of the galaxy, so a measurement of their intensity will have far reaching effects on theories of the interstellar gas and the evolution of the galaxy. The nuclear abundances, and in particular the presence or absence of high Z nuclei, will give critical information on the proximity of cosmic ray sources.This is one of the publications by the Science Advisory Group.     

Cosmic-ray acceleration and transport,and diffuse galactic γ-ray emission

Cosmic-ray acceleration and transport is considered from the point of view of application to diffuse galactic -ray sources. As an introduction we review several source models, in particular supernovae exploding inside or near large interstellar clouds. The complex problem of cosmic ray transport in random electromagnetic fields is reduced to three cases which should be sufficient for practical purposes. As far as diffusive acceleration is concerned, apart from reviewing the basic physical principles, we point out the relation between shock acceleration and 2nd order Fermi acceleration, and the relative importance of the two processes around interstellar shock waves. For -ray source models the interaction of cosmic rays with dense clouds assumes great importance. Past discussions had been confined to static interactions of clouds with the ambient medium in the sense that no large scale mass motions in the ambient interstellar medium were considered. The well-known result then is that down to some tens of MeV or less, cosmic-ray nucleons should freely penetrate molecular clouds of typical masses and sizes. The self-exclusion of very low energy nucleons however may affect electron transport with consequences for the Bremsstrahlung -luminosity of such clouds.In this paper we consider also the dynamical interaction of dense clouds with a surrounding hot interstellar medium. Through cloud evaporation and accretion there exist mass flows in the cloud surroundings. We argue that in the case of (small) cloud evaporation the galactic cosmic rays will be essentially excluded from the clouds. The dynamic effects of cosmic rays on the flow should be minor in this case. For the opposite case of gas accretion onto (large) clouds, cosmic-ray effects on the flow will in general be large, limiting the cosmic-ray compression inside the cloud to dynamic pressure equilibrium. This should have a number of interesting and new consequences for -ray astronomy. A first, qualitative discussion is given in the last section.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.     

New Experimental Data and What It Tells Us About the Sources and Acceleration of Cosmic Rays

This paper summarizes new data in several fields of astronomy that relate to the origin and acceleration of cosmic rays in our galaxy and similar nearby galaxies. Data from radio astronomy shows that supernova remnants, both in our galaxy and neighboring galaxies, appear to be the sources of most of the accelerated electrons observed in these galaxies. -ray measurements also reveal several strong sources associated with supernova remnants in our galaxy. These sources have -ray spectra that are suggestive of the acceleration of cosmic-ray nuclei. Cosmic-ray observations from the Voyager and Ulysses spacecraft suggest a source composition that is very similar to the solar composition but with distinctive differences in the ⁴He, ¹²C,¹⁴ N and ²²Ne abundances that are the imprint of giant W-R star nucleosynthesis. Injection effects which depend on the first ionization potential (FIP) of the elements involved are also observed, in a manner similar to the fractionization observed between the solar photosphere and corona and also analogous to the preferential acceleration observed for high FIP elements at the heliospheric solar wind termination shock. Most of the ⁵⁹Ni produced in the nucleosynthesis of Fe peak nuclei just prior to a SN explosion appears to have decayed to ⁵⁹Co before the cosmic rays have been accelerated, suggesting that the⁵⁹ Ni is accelerated at least 10⁵ yr after it is produced. The decay of certain K capture isotopes produced during cosmic-ray propagation has also been observed for the first time. These measurements suggest that re-acceleration after an initial principal acceleration cannot be large. The high energy spectral indices of cosmic-ray nuclei show a significant charge dependent trend with the index of hydrogen being -2.76 and that of Fe -2.61. The escape length dependence of cosmic rays from our galaxy can now be measured up to ~300 GeV nucl^-1 using the Fe sec/Fe ratio. This escape length is P ^-0.05 above 10 GeV nucl^-1 leading to a typical source spectral index of (2.70±0.10) -0.50 = -2.20 for nuclei. This is similar to the source index of -2.3 inferred for electrons within the errors of ±0.1 in the index for both components. Spacecraft measurements in the outer heliosphere suggest that the local cosmic-ray energy density is ~2eV cm^-3 – larger than previously assumed. Gamma-ray measurements of electron bremsstrahlung below 50 MeV from the Comptel experiment on CGRO show that fully 20–30% of this energy is in electrons, several times that previously assumed. New estimates of the amount of matter traversed by cosmic rays using measurements of the B/C ratio are also higher than earlier estimates – this value is now ~10 g cm^-2 at 1 GeV nucl^-1. Thus altogether cosmic rays are energetically a more important component of our galaxy than previously assumed. This has implications both for the types of sources that are capable of accelerating cosmic rays and also for the role that cosmic rays may play in ionizing the diffuse interstellar medium.     

Cosmic-Ray Energy Spectra and Time Variations in the Local Interstellar Medium: Constraints and Uncertainties

The spectra of galactic cosmic rays that are observed inside the heliosphere result from the interaction of the spectra present in the local interstellar medium with the structured but turbulent magnetic field carried by the solar wind. Observational tests of solar modulation theory depend on comparisons between spectra inside and outside the heliosphere. Our knowledge of the local interstellar spectra are indirect, using extrapolations of interplanetary spectra measured at high energies where solar modulation effects are minimal and modeling of the physical processes that occur during particle acceleration and transport in the interstellar medium. The resulting estimates of the interstellar spectra can also be checked against observations of the effects that cosmic rays have on the chemistry of the interstellar medium and on the production of the diffuse galactic gamma-ray background. I review the present understanding of the local galactic cosmic-ray spectra, emphasizing the constraints set by observations and the uncertainties that remain.     

Magnetic Fields in Galactic Haloes

Magnetic fields on a range of scales play a large role in the ecosystems of galaxies, both in the galactic disk and in the extended layers of gas away from the plane. Observing magnetic field strength, structure and orientation is complex, and necessarily indirect. Observational data of magnetic fields in the halo of the Milky Way are scarce, and non-conclusive about the large-scale structure of the field. In external galaxies, various large-scale configurations of magnetic fields are measured, but many uncertainties about exact configurations and their origin remain. There is a strong interaction between magnetic fields and other components in the interstellar medium such as ionized and neutral gas and cosmic rays. The energy densities of these components are comparable on large scales, indicating that magnetic fields are not passive tracers but that magnetic field feedback on the other interstellar medium components needs to be taken into account.     

COS-B views on the diffuse galactic gamma-ray emission and some point sources

The correlation between diffuse galactic gamma rays and gas tracers is studied using the final COS-B database and H i and CO surveys covering the entire galactic plane. A good quantitative fit to the gamma rays is obtained, with a small galacto-centric gradient in the gamma-ray emissivity per hydrogen atom. The average ratio of H₂ column density to integrated CO temperature is determined, the best estimate being (2.3 ± 0.3) × 10² molecules cm^–2 (K km s^–1)^–1. Strictly taken, this value is an upper limit. The corresponding mass of molecular hydrogen in the inner galaxy, derived using both 1st and 4th quadrants, is 1.0 × 10⁹ M .The softer gamma-ray spectrum towards the inner galaxy found in previous work can be attributed to a steeper emissivity gradient at low energies and/or to a softer gamma-ray spectrum of the emission distributed like molecular gas. A steeper emissivity gradient at low energies could be related to cosmic-ray spectral variations in the Galaxy, to different distributions of cosmic-ray electrons and nuclei, or to a contribution from discrete sources. A softer spectrum for the emission associated with molecular clouds may be physically related to the clouds themselves (i.e., cosmic-ray spectral variations) or to an associated discrete source distribution.New results on the temporal and spectral characteristics of the high-energy (50 MeV to 5 GeV) gammaray emission from the Vela pulsar are presented. The whole pulsed flux is found to exhibit long-term variability. Five discrete emission regions within the pulsar lightcurve have been identified, with the spectral characteristics and long-term behaviour being different. These characteristics differ significantly from those reported earlier for the Crab pulsar. However, geometrical pulsar models have been proposed (e.g., Morini, 1983; Smith, 1986) which could explain many of these features.     

Cosmic-ray Composition as Observed by Ulysses

The cosmic ray isotopic composition measurements from the High Energy Telescope (HET) on the Ulysses spacecraft are reviewed. The source isotopic composition of key elements is found to be surprisingly like the Solar system abundances with the notable exception of ²²Ne. The average density of interstellar material cosmic rays traverse is found to be 0.25 atom cm^–3, corresponding to a confinement time of 20 Myr. Vanadium isotopic abundances are shown to be consistent with weak cosmic-ray reacceleration. The implications of these measurements are discussed.     

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.     

The isotopic composition of anomalous cosmic rays from sampex

Measurements of the anomalous cosmic ray (ACR) isotopic composition have been made in three regions of the magnetosphere accessible from the polar Earth orbit of SAMPEX, including the interplanetary medium at high latitudes and geomagnetically trapped ACRs. At those latitudes where ACRs can penetrate the Earth's magnetic field while fully stripped galactic cosmic rays (GCRs) of similar energies are excluded, a pure ACR sample is observed to have the following composition: ¹⁵N/N < 0.023, ¹⁸O/¹⁶O < 0.0034, and ²²Ne/²⁰Ne = 0.077(+0.085, –0.023). We compare our values with those found by previous investigators and with those measured in other samples of solar and galactic material. In particular, a comparison of ²²Ne/²⁰Ne measurements from various sources implies that GCRs are not simply an accelerated sample of the local interstellar medium.     

Large-scale structure of the Galaxy and high-energy gamma-ray observations

Detailed information on the high-energy gamma-ray emission from our Galaxy has become available through the two dedicated satellite missions SAS-2 and COS-B. The consistency of the two datasets is discussed; while a satisfying general agreement is observed, a few distinct discrepancies point to possible time variations within the compact source component of the total galactic emission. The bulk of emission appears very well correlated to the column density of the total interstellar gas, as traced by radio observations of Hi and CO. The gamma-ray observations exclude the possibility that H₂ dominates in the inner Galaxy, its mass should not exceed the mass existing in the form of Hi. Neither a significant galactocentric gradient of the (high-energy) cosmic-ray flux density is suggested inside the solar circle (outside a decrease is needed), nor a linear coupling between the cosmic rays and the gas is indicated by the gamma-ray data. The systematic variation with longitude of the spectrum of the gamma-ray emission points to an increased flux of cosmic-ray electrons in the 100 MeV to 1 GeV energy range in regions where dense clouds are concentrated. The variation could as well be due to the largely unresolved population of compact gamma-ray objects.     

The termination shock of the solar wind

An overview of the solar wind termination shock is presented including: its place in the heliosphere and its origin; its structure including the role of interstellar pickup ions and galactic and anomalous cosmic rays; its inferred location based on Lyman- backscatter, Voyager radio signals, and anomalous cosmic rays; its shape and movement.     

Cosmic ray investigation for the Voyager missions; energetic particle studies in the outer heliosphere—And beyond

A cosmic-ray detector system (CRS) has been developed for the Voyager mission which will measure the energy spectrum of electrons from 3–110 MeV and the energy spectra and elemental composition of all cosmic-ray nuclei from hydrogen through iron over an energy range from 1–500 MeV/nuc. Isotopes of hydrogen through sulfur will be resolved from 2–75 MeV/nuc. Studies with CRS data will provide information on the energy content, origin and acceleration process, life history, and dynamics of cosmic rays in the galaxy, and contribute to an understanding of the nucleosynthesis of elements in the cosmic-ray sources. Particular emphasis will be placed on low-energy phenomena that are expected to exist in interstellar space and are known to be present in the outer Solar System. This investigation will also add to our understanding of the transport of cosmic rays, Jovian electrons, and low-energy interplanetary particles over an extended region of interplanetary space. A major contribution to these areas of study will be the measurement of three-dimensional streaming patterns of nuclei from H through Fe and electrons over an extended energy range, with a precision that will allow determination of anisotropies down to 1%. The required combination of charge resolution, reliability and redundance has been achieved with systems consisting entirely of solid-state charged-particle detectors.Principal Investigator of the Voyager Cosmic Ray Experiment.     

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.     

Cosmic Rays in Galactic and Extragalactic Magnetic Fields

We briefly review sources of cosmic rays, their composition and spectra as well as their propagation in the galactic and extragalactic magnetic fields, both regular and fluctuating. A special attention is paid to the recent results of the X-ray and gamma-ray observations that shed light on the origin of the galactic cosmic rays and the challenging results of Pierre Auger Observatory on the ultra high energy cosmic rays. The perspectives of both high energy astrophysics and cosmic-ray astronomy to identify the sources of ultra high energy cosmic rays, the mechanisms of particle acceleration, to measure the intergalactic radiation fields and to reveal the structure of magnetic fields of very different scales are outlined.     

The relevance of molecular observations to gamma-ray astronomy

The interaction of cosmic rays with interstellar clouds may produce some of the observed gamma-ray sources. The use of molecular observations to estimate the cloud masses, which are used to derive cosmic-ray fluxes, is reviewed. Molecular diagnostics of high cosmic-ray ionization rates are discussed, and a detailed application of those diagnostics is summarised and presented as evidence that second-order Fermi acceleration is important in old supernova remnants and can produce cosmic rays of too low energy to induce gamma-ray emission.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.Royal Society Jaffé Donation Fellow.     

Multi-Scale Plasma Turbulence in the Diffuse Interstellar Medium

I discuss how radioastronomical observations can provide information on the turbulence that governs the propagation of cosmic rays in the Galaxy. Interstellar radio wave propagation effects, collectively referred to as interstellar scintillations, yield information on the spatial power spectra of fluctuations in plasma density and magnetic field. Results of relevance to cosmic-ray physics are the existence of interstellar turbulence over a wide range of spatial scales (which can thus interact with a wide range of cosmic ray energies), the detection of magnetic field fluctuations in association with this turbulence, and a change in the nature of the turbulence on spatial scales of about 3.5 parsecs. A number of mysteries remain, such as the apparent suppression of Fast Magnetosonic wave generation by the interstellar turbulence.     

Cosmic-ray scintillations and dynamic processes in space

Cosmic-ray scintillations registered by ground-base observations reflect, as a rule, the action of a whole number of processes proceeding in interplanetary space and Earth's magnetosphere. The study of scintillation phenomena in cosmic rays, is, in fact, divided into a number of problems connected with the interaction of charged particles of cosmic radiation with the matter and fields which they encounter in the entire length of their propagation. The cosmic-ray scintillations established by different authors from the data of ground-base and high-altitude devices for quiet and disturbed periods, as well as the theoretical calculations of different models and mechanisms of the origin and development of cosmic-ray scintillations are analyzed. High-frequency scintillations of f 10^-5 Hz are shown to be precursors of an approaching shock wave, scintillations with periods of the order of 10–20 and 40–50 min being most sensitive to disturbances of interplanetary medium near the Earth. Since cosmic rays of different energies are sensitive to different processes in interplanetary space at different distances from the Earth, one can sound the conditions in interplanetary medium up to 10¹⁵ cm from the Earth by measuring particle fluxes at different energy ranges.     

Nonthermal radio emission from the Galaxy

Synchrotron radio emission from interstellar space has long been recognized as a useful tool to probe into the galactic distribution of high energy electrons and magnetic fields. We first review the results obtained from the local (<2kpc distant) region of the Galaxy and conclude that the observed local synchrotron emissivity is consistently explained by the measured cosmic ray electron spectrum and the interstellar magnetic field if some reasonable assumptions are allowed. The large scale distribution of radio emissivity shows evidence for spiral structure and is likely to originate in two distinct disk systems: a thin disk (thickness 250 pc in the inner Galaxy) formed by population I objects which emits about 10% of the galactic radio luminosity and a thick disk (2.5 kpc thick in the inner Galaxy) which constitutes the truly diffuse emission and produces 90% of the total luminosity.