Propagation Of Galactic Cosmic Rays Research Articles

Models for galactic cosmic-ray propagation

A new numerical model of particle propagation in the Galaxy has been developed, which allows the study of cosmic-ray and gamma-ray production and propagation in 2D or 3D, including a full reaction network. This is a further development of the code which has been used for studies of cosmic ray reacceleration, Galactic halo size, antiprotons and positrons in cosmic rays, the interpretation of diffuse continuum gamma rays, and dark matter. In this paper we illustrate recent results focussing on B/C, sub-Fe/Fe, ACE radioactive isotope data, source abundances and antiprotons. From the radioactive nuclei we derive a range of 3–7 kpc for the height of the cosmic-ray halo.

Scattering of energetic particles by anisotropic magnetohydrodynamic turbulence with a goldreich-sridhar power spectrum

Scattering rates for a Goldreich-Sridhar (GS) spectrum of anisotropic, incompressible, magnetohydrodynamic turbulence are calculated in the quasilinear approximation. Because the small-scale fluctuations are constrained to have wave vectors nearly perpendicular to the background magnetic field, scattering is too weak to provide either the mean-free paths commonly used in Galactic cosmic-ray propagation models or the mean-free paths required for acceleration of cosmic rays at quasiparallel shocks. Where strong pitch-angle scattering occurs, it is due to fluctuations not described by the GS spectrum, such as fluctuations generated by streaming cosmic rays.

The Modified Weighted Slab Technique: Models and Results

In an attempt to understand the source and propagation of Galactic cosmic rays, we have employed the modified weighted slab technique along with recent values of the relevant cross sections to compute primary to secondary ratios including B/C and sub-Fe/Fe for different Galactic propagation models. The models that we have considered are the disk-halo diffusion model, the dynamical halo wind model, the turbulent diffusion model, and a model with minimal reacceleration. The modified weighted slab technique will be briefly discussed and a more detailed description of the models will be given. We will also discuss the impact that the various models have on the problem of anisotropy at high energy and discuss what properties of a particular model bear on this issue.

Total charge-changing cross sections for neutron-rich light nuclei

Total charge-changing cross sections at relativistic energies on a carbon target have been measured for the light stable and neutron-rich radioactive nuclei 14 Be, 10−19 B, 12−20 C, 14−23 N, 16−24 O, and 18−27 F. A combined analysis of interaction and total charge-changing cross sections allows to draw definite conclusions concerning the thickness of the neutron skins or the size of the neutron halos for very neutron-rich isotopes. The obtained cross sections are also important in astrophysical applications to describe the propagation of galactic cosmic rays through the interstellar medium. A simple parameterization can reproduce the total charge-changing cross section within an accuracy of 5% for light nuclei from the valley of β -stability up to the drip line. These systematics improve the predictive capability of the formulae used to describe the unknown cross sections.

Confinement and Isotropization of Galactic Cosmic Rays by Molecular‐Cloud Magnetic Mirrors When Turbulent Scattering Is Weak

Theoretical studies of magnetohydrodynamic (MHD) turbulence and observations of solar wind —uc- tuations suggest that MHD turbulence in the interstellar medium is anisotropic at small scales, with smooth variations along the background magnetic —eld and sharp variations perpendicular to the back- ground —eld. Turbulence with this anisotropy is inefficient at scattering cosmic rays, and thus the scat- tering rate l may be smaller than has been traditionally assumed in diUusion models of Galactic cosmic-ray propagation, at least for cosmic-ray energies E above 1011¨1012 eV at which self-con—nement is not possible. In this paper, it is shown that Galactic cosmic rays can be eUectively con—ned through magnetic re—ection by molecular clouds, even when turbulent scattering is weak. Elmegreens quasi- fractal model of molecular-cloud structure is used to argue that a typical magnetic —eld line passes through a molecular cloud complex once every D300 pc. Once inside the complex, the —eld line will in most cases be focused into one or more dense clumps in which the magnetic —eld can be much stronger than the average —eld in the intercloud medium (ICM). Cosmic rays following —eld lines into cloud com- plexes are most often magnetically re—ected back into the ICM, since strong-—eld regions act as magnetic mirrors. For a broad range of cosmic-ray energies, a cosmic ray initially following some particular —eld line separates from that —eld line sufficiently slowly that the cosmic ray can be trapped between neigh- boring cloud complexes for long periods of time. The suppression of cosmic-ray diUusion due to mag- netic trapping is calculated in this paper with the use of phenomenological arguments, asymptotic analysis, and Monte Carlo particle simulations. Formulas for the coefficient of diUusion perpendicular to the Galactic disk are derived for several diUerent parameter regimes within the E-l plane. In one of these parameter regimes in which scattering is weak, it is shown that molecular-cloud magnetic mirrors strongly reduce cosmic-ray anisotropy in the ICM, and analytic formulas for the angular harmonics are derived. Subject headings: acceleration of particlescosmic raysISM: cloudsISM: magnetic —elds ¨ turbulence

Monte-Carlo approach to galactic cosmic ray propagation in the heliosphere

In the present paper we consider a possibility of using stochastic simulation (Monte-Carlo) technique approach to the study of Galactic Cosmic Ray propagation in the Heliosphere. We developed a technique for calculation of the cosmic ray propagation in a spherically symmetric steady state approximation of the Heliosphere. A comparison of the calculation results with those obtained by other methods as well as with an analytical approximation shows a good agreement. Besides, in the frameworks of the approximation used, we calculated the solar modulation of monoenergetic fluxes of Galactic Cosmic Rays entering the Heliosphere, in the particle's energy range 0.1–15 GeV. We studied the details of the modulation in their dependence of the initial particle's energy. In particular, a linear scaling of particle's energy losses vs. diffusion time is shown.

A Markov Stochastic Process Theory of Cosmic‐Ray Modulation

This paper introduces the use of Markov stochastic process theory to study heliospheric modulation of cosmic rays. The basic cosmic-ray transport equation is reformulated with a set of stochastic differential equations that describe the guiding center and momentum of individual charged particles randomly walking in the heliospheric magnetic field. The Fokker-Planck diffusion equation for the cosmic-ray isotropic distribution function can be derived from these stochastic differential equations of basic cosmic-ray transport. General exact solutions to the initial-boundary value problems of the Fokker-Planck diffusion equation are obtained in terms of time-backward Markov stochastic processes. It is demonstrated with a few examples that modulated cosmic-ray spectra can be calculated with Monte-Carlo simulation of stochastic processes, and the results from the stochastic process simulation are completely consistent with those from calculations that numerically solve the Fokker-Planck diffusion equation. In addition to the capability of modulation spectrum calculation, the stochastic process simulation reveals new information about the behavior of individual particles during their transit through the heliospheric magnetic field, which includes the trajectory of particles, momentum-loss history, source particle distribution as functions of location and momentum, and distribution of transport time or path length, all of which are normally not available by just solving the Fokker-Planck diffusion equation. This method will allow us to tackle very complicated time-dependent cosmic-ray modulation problems that are impossible by numerically solving the Fokker-Planck diffusion equation. Diffusive particle acceleration and interstellar propagation of Galactic cosmic rays may also be studied with this stochastic process method.

Modulation of Galactic Cosmic Rays at Solar Minimum

Cosmic ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence and therefore provide a unique probe for conditions in the changing heliosphere. During the last four years, concentrated around the solar minimum period of solar cycle 22, the exploration of the solar polar regions by the joint ESA/NASA mission Ulysses revealed the three-dimensional behavior of cosmic rays in the inner and middle heliosphere. Also during the last decades, the Pioneer and Voyager missions have greatly expanded our understanding of the structure and extent of the outer heliosphere. Simultaneously, numerical models describing the propagation of galactic cosmic rays are becoming sophisticated tools for interpreting and understanding these observations. We give an introduction to the subject of the modulation of galactic cosmic rays in the heliosphere during solar minimum. The modulation effects on cosmic rays of corotating interaction regions and their successors in the outer heliosphere are discussed in more detail by Gazis, McDonald et al. (1999) and McKibben, Jokipii et al. (1999) in this volume. Cosmic-ray observations from the Ulysses spacecraft at high heliographic latitudes are also described extensively in this volume by Kunow, Lee et al. (1999).

Measurement of charge changing and isotopic cross sections at ∼600 MeV/nucleon from the interactions of ∼30 separate beams of relativistic nuclei from10Bto55Mnin a liquid hydrogen target

We report the measurement of secondary charge and isotopic fragmentation cross sections in a liquid hydrogen target from 30 incident beams of relativistic nuclei ranging from ${}^{10}\mathrm{B}$ to ${}^{55}\mathrm{Mn}.$ These individual beams were obtained by initially accelerating 580 MeV/nucleon ${}^{40}\mathrm{Ar}$ and 630 MeV/nucleon ${}^{56}\mathrm{Fe}$ nuclei and letting these nuclei interact in a thin ${\mathrm{CH}}_{2}$ target in the beam line. The fragments of these interactions were then focused according to their $A/Z$ ratios onto a hydrogen target and the charge and isotopic composition of the fragmentation in this target was measured using our standard cosmic ray telescope. Several of these nuclei have had their cross sections measured previously and a comparison with earlier data confirms the estimated precision \ensuremath{\sim}5% of the new cross section data. The 30 nuclei for which the cross sections were measured doubles the previously reported data for 15 nuclei from several experiments in the charge range from Li to Ni. The systematics of these new cross sections are discussed both with respect to the charge changing and isotopic cross sections. These systematics will lead to improvements in the productive capability of the formulas used to describe the unmeasured cross sections. It should be noted, however, that from the point of view of the propagation of galactic cosmic rays through the interstellar medium, which is one of the main goals of this experiment, the fragmentation cross sections have now been measured at at least one energy for over 98% of the arriving particles with $Z=3--28.$

Production and Propagation of Cosmic‐Ray Positrons and Electrons

We have made a new calculation of the cosmic-ray secondary positron spectrum using a diffusive halo model for Galactic cosmic-ray propagation. The code computes self-consistently the spectra of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. The models are first adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the interstellar proton and Helium spectra are then computed; these spectra are used to obtain the source function for the secondary positrons/electrons which are finally propagated with the same model parameters. The primary electron spectrum is evaluated, again using the same model. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. Our study includes a critical re-evaluation of the secondary decay calculation for positrons. The predicted positron fraction is in good agreement with the measurements up to 10 GeV, beyond which the observed flux is higher than that calculated. Since the positron fraction is now accurately measured in the 1-10 GeV range our primary electron spectrum should be a good estimate of the true interstellar spectrum in this range, of interest for gamma ray and solar modulation studies. We further show that a harder interstellar nucleon spectrum, similar to that suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV, can reproduce the positron observations above 10 GeV without requiring a primary positron component.

The effect of magnetic helicity on the propagation of galactic cosmic rays

Apart from particle drifts, magnetic helicity is the only other known phenomenon that may affect the propagation of galactic cosmic rays in a charge-dependent manner. The detection of steady magnetic helicity in low-frequency interplanetary magnetic field turbulence (Smith and Bieber, 1993) justifies its inclusion in modulation models. In the current model, helicity is introduced through its effect on diffusion parallel to the mean magnetic field. The spatial variation of the parallel mean free path is discussed and it is shown that the largest charge-dependent effect caused by helicity occurs over the solar poles. The transport equation for galactic cosmic rays is solved numerically to illustrate the effect of helicity on particle fluxes. It is shown that helicity causes higher particle intensities during solar cycles with negative polarity and lower intensities during cycles with positive polarity.

A Solar Polar North-South Asymmetry for Cosmic-Ray Propagation in the Heliosphere: The [ITAL]Ulysses[/ITAL] Pole-to-Pole Rapid Transit

The pole-to-pole fast transit of the heliosphere at ~2 AU from the Sun by the international Ulysses spacecraft has made it possible for the first time to investigate in three dimensions the solar wind, interplanetary magnetic field, and—as reported herein—the global propagation of Galactic cosmic rays and anomalous helium. We use measurements of cosmic-ray protons with a mean energy ~2 GeV and helium component fluxes in the energy range 30-70 MeV per nucleon to illustrate new phenomena of charged particle propagation through the heliosphere. The latitude gradients are the same in both hemispheres—namely, ~0.3% deg-1 for cosmic rays, and ~0.7% deg-1 for anomalous helium. It was a surprise to discover that, although the heliospheric modulation of these components is remarkably symmetric globally, the plane of symmetry is offset southward from the Sun's heliographic equator by ~10° of latitude and that the fluxes over the north pole exceed those at the south pole by ~6%-15%. Although unexplained at present, the implications of this offset are discussed with respect to interplanetary magnetic field and solar wind observations.

Propagation of galactic cosmic rays under diffusive reacceleration

view Abstract Citations (62) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Propagation of Galactic Cosmic Rays under Diffusive Reacceleration Heinbach, U. ; Simon, M. Abstract The propagation of cosmic rays in the galaxy is understood as a diffusive process where the magnetic field irregularities serve as scattering center. In this paper we pursue the idea that particles which interact with turbulent magnetic fields should naturally also gain energy due to a stochastic second-order Fermi type of acceleration. This link between diffusion and reacceleration is based on general physical principles, and we can show that the cosmic-ray data can be explained in this 'Diffusive Reacceleration Model.' As a consequence, the escape length lambdaesc(E), which defines the particles path in the galaxy, becomes a power-law in rigidity with an exponent of -1/3, which would indicate a Kolmogorov-type spectrum of magnetic turbulences. The increase of lambdaesc(E) combined with the increase of the amount of reacceleration to low energies in then interpreted as a decrease of the diffusion coefficient or an increase of the frequencies of scattering center. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175350 Bibcode: 1995ApJ...441..209H Keywords: Astronomical Models; Cosmic Rays; Diffusion; Particle Acceleration; Turbulence; Kolmogorov Theory; Magnetohydrodynamics; Mathematical Models; Radiative Transfer; Stochastic Processes; Transport Theory; Astrophysics; ACCELERATION OF PARTICLES; DIFFUSION; ISM: COSMIC RAYS; MAGNETOHYDRODYNAMICS: MHD; TURBULENCE full text sources ADS |

New measurements of charge-changing cross sections in carbon and hydrogen targets above 2 GeV per nucleon: Evidence for an energy dependence that may strongly affect estimates of the energy dependence of cosmic-ray diffusion in the galaxy

view Abstract Citations (5) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Measurements of Charge-changing Cross Sections in Carbon and Hydrogen Targets above 2 GeV per Nucleon: Evidence for an Energy Dependence That May Strongly Affect Estimates of the Energy Dependence of Cosmic-Ray Diffusion in the Galaxy Webber, W. R. ; Binns, W. R. ; Crary, D. ; Westphall, M. Abstract We have recently measured the fragmentation of approximately 15 GeV per nucleon Si nuclei in CH2 and C targets and, by subtraction, H targets, at Brookhaven. The total-changing cross section for Si is larger than we measured earlier at approximately 1.3 GeV per nucleon. At the same time the partial cross sections for charge changes of 1 and 2 are approximately 20% to 50% smaller for all targets including H. An overall systematic behavior is observed for the ratio of the 15 GeV per nucleon to 1.3 GeV per nucleon partial cross sections as a function of the magnitude of the charge change. These results, when coupled with earlier measurements of the fragmentation of high energy O right arrow N and Si right arrow Al by the Siegen group (Hirzebruch et al. 1991), which also show a decrease in the partial cross sections for charge changes of 1 and 2 above 1 to 2 GeV per nucleon in H targets, have major implications for the energy dependence of the propagation of Galactic cosmic rays. This is because the B/C ratio is used to determine this dependence and the main channel for B production, C right arrow B, should also show a decreasing cross section with increasing energy according to the above argument. We will present this new cross-section data and examine their implications for the propagation of cosmic rays. Publication: The Astrophysical Journal Pub Date: July 1994 DOI: 10.1086/174360 Bibcode: 1994ApJ...429..764W Keywords: Carbon; Charge Exchange; Cosmic Rays; Galaxies; Hydrogen; Ionization Cross Sections; Particle Acceleration; Particle Diffusion; Energy Distribution; Position Sensing; Proportional Counters; Scintillation Counters; Silicon; Astrophysics; ACCELERATION OF PARTICLES; ATOMIC DATA; ATOMIC PROCESSES; ISM: COSMIC RAYS full text sources ADS |

Transport of solar energetic particles

view Abstract Citations (36) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Transport of Solar Energetic Particles Droege, Wolfgang Abstract New developments in the understanding of the interplanetary transport of solar cosmic rays are reviewed. Based on carefully analyzed solar particle events observed on the Helios and International Sun Earth Explorer (ISEE) 3 spacecraft the relation of transport parameters to the structure of the interplanetary magnetic field is discussed. Special emphasis is given to a comparison of particle mean free paths determined from fits to intensity and anisotropy profiles with theoretical predictions derived from magnetic field spectra measured at the time of the solar particle event. Different aspects of the turbulence and wave models for the magnetic fluctuations are considered, including the effects resulting from the finite temperature of the plasma and of resonance broadening. It is found that a modified quasi-linear theory of particle scattering taking into account the effects of plasma waves propagating with respect to the average solar wind flow and the proper treatment of the dispersion relation at high wavenumber gives results which are in several cases in good agreement with particle observations in the interplanetary medium between 0.3 and 1 AU, indicating that quasi-linear theory is probably a good approximation to a full theory of solar particle transport. This has important implications for other astrophysical problems where quasi-linear theory is often used, such as the propagation and acceleration of Galactic cosmic rays and particle acceleration at shock waves. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1994 DOI: 10.1086/191876 Bibcode: 1994ApJS...90..567D Keywords: Energetic Particles; Interplanetary Magnetic Fields; Interplanetary Medium; Particle Acceleration; Scatter Propagation; Solar Cosmic Rays; International Sun Earth Explorers; Transport Theory; Turbulence Models; Wave Propagation; Solar Physics; ACCELERATION OF PARTICLES; INTERPLANETARY MEDIUM; ISM: COSMIC RAYS; MAGNETOHYDRODYNAMICS: MHD; SUN: PARTICLE EMISSION; SUN: SOLAR WIND full text sources ADS |

Transport of Solar Energetic Particles

AbstractNew developments in the understanding of the interplanetary transport of solar cosmic rays are reviewed. Based on carefully analyzed solar particle events observed on the Helios and ISEE 3 spacecraft, the relation of transport parameters to the structure of the interplanetary magnetic field is discussed. Special emphasis is given to a comparison of particle mean free paths determined from fits to intensity and anisotropy profiles with theoretical predictions derived from magnetic field spectra measured at the time of the solar particle event. Different aspects of the turbulence and wave models for the magnetic fluctuations are considered, including the effects resulting from the finite temperature of the plasma and of resonance broadening. It is found that a modified quasi-linear theory of particle scattering taking into account the effects of plasma waves propagating with respect to the average solar wind flow and the proper treatment of the dispersion relation at high wavenumber gives results which are in several cases in good agreement with particle observations in the interplanetary medium between 0.3 and 1 AU, indicating that quasi-linear theory is probably a good approximation to a full theory of solar particle transport. This has important implications for other astrophysical problems where quasi-linear theory is often used, such as the propagation and acceleration of Galactic cosmic rays and particle acceleration at shock waves.Subject headings: acceleration of particles — cosmic rays — interplanetary medium — MHD — solar wind — Sun: particle emission

Production and evolution of LiBeB isotopes in the galaxy

We reassess the problem of the production of the light elements lithium, beryllium, and boron, by energetic collisions between Galactic cosmic rays (GCR) and interstellar gas nuclei, in the framework of a consistent model for the chemical evolution of our Galaxy. We propose a new scenario for the propagation of GCR in the early Galaxy suggesting that, during its halo phase, GCR were more efficiently confined than today and had flatter spectra at low energy

Overview of Cosmic Rays, Solar and Interplanetary Physics Research (1987 – 1990)

A brief survey of recent U.S. investigations in the field of heliospheric plasmas and their manifestations is presented, introducing the following collection of detailed reviews (accessions A91-46959 to A91-46964). Topics examined include the large-scale structure of interplanetary plasmas, models of Galactic cosmic-ray production and propagation, solar-wind turbulence, long-period solar-terrestrial variability, the possible relation between solar-neutrino counts and the sunspot cycle, X-ray studies of solar flares and their implications for solar processes, and the near-sun magnetic field.

Measurement of the cosmic-ray iron spectrum between 60 and 200 GeV per nucleon

view Abstract Citations (3) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measurement of the cosmic-ray iron spectrum between 60 and 200 GeV per nucleon Esposito, Joseph A. ; Streitmatter, Robert E. ; Balasubrahmanyan, V. K. ; Ormes, Jonathan F. Abstract A measurement of the spectral index of Galactic cosmic-ray (GCR) iron has been made using a high-energy gas Cerenkov spectrometer. The spectral index of GCR iron is found to be 2.56 + or - 0.11 in the energy range 57-200 GeV/ nucleon. This result indicates that the source spectrum of GCR iron is similar to that of other primary GCR nuclei and is consistent with the simplest models of GCR propagation. Publication: The Astrophysical Journal Pub Date: March 1990 DOI: 10.1086/168483 Bibcode: 1990ApJ...351..459E Keywords: Cosmic Rays; Galactic Radiation; Nucleons; Cerenkov Counters; Iron; Radiation Measurement; Space Radiation full text sources ADS |

In Situ-Produced Cosmogenic Isotopes in Terrestrial Rocks

In the late forties, Libby and his collaborators detected the naturally occurring radioactive isotope 14C, produced by cosmic radiation in the Earth's atmosphere (Libby 1946, Anderson et al 1 947). This marked the onset of the search for cosmic-ray-produced (cosmogenic) isotopic changes in terrestrial and extraterrestrial samples and in the cosmic rays themselves. During the four decades since Libby's discovery, some four dozen cosmo­ genic stable and radioactive isotopes in extraterrestrial samples (Reedy et al 1983) and two dozen in terrestrial samples (Lal & Peters 1 967) have been discovered. These isotopes have found applications in cosmic-ray physics (in the study of acceleration and propagation of galactic and solar cosmic rays), in solar system astrophysics (in determining the evolutionary history of meteorites, the lunar surface, and interplanetary dust particles), and in the Earth sciences (in archaeology, meteorology, glaciology and oceanography). For reviews on these topics, reference is made to Simpson (1983); Reedy et al (1983); and Lal & Peters (1967), Oeschger et al (1970), and Faure ( 1986), respectively. The process of discovery continues, pro­ pelled by advances in the techniques used to measure these isotopes. Even now, examples of new isotopic effects are being discovered, and new applications are being made of the various isotopic effects. When one looks at the history of the development of the terrestrial cosmogenic field, one observes that it grew in waves: After an idea is implemented, the field