Galactic Cosmic Ray Transport Research Articles

Latitude effect of muons in the Earth’s atmosphere during solar activity minimum

The results of muon flux measurements (E ≥ 70 MeV) in the atmosphere during solar activity minimum are reported. The measurements were performed during Antarctic sea expedition in 1975–1976 at several geomagnetic locations (from 1 to 14 GV). The experimental data obtained made it possible to determine the latitude effect of muons in the distribution at different levels in the atmosphere. The Geant4 program was used to simulate the galactic cosmic ray transport in the Earth’s atmosphere and evaluate the angular and spectral distributions (p, e−/e+, photons, muons) at different atmospheric levels. The experimental and simulated results are in satisfactory agreement.

A Markov Chain Monte Carlo technique to sample transport and source parameters of Galactic cosmic rays

Context. Propagation of charged cosmic-rays in the Galaxy depends on the transport parameters, whose number can be large depending on the propagation model under scrutiny. A standard approach for determining these parameters is a manual scan, leading to an inefficient and incomplete coverage of the parameter space. Aims. In analyzing the data from forthcoming experiments, a more sophisticated strategy is required. An automated statistical tool is used, which enables a full coverage of the parameter space and provides a sound determination of the transport and source parameters. The uncertainties in these parameters are also derived. Methods. We implement a Markov Chain Monte Carlo (MCMC), which is well suited to multi-parameter determination. Its specificities (burn-in length, acceptance, and correlation length) are discussed in the context of cosmic-ray physics. Its capabilities and performances are explored in the phenomenologically well-understood Leaky-Box Model. Results. From a technical point of view, a trial function based on binary-space partitioning is found to be extremely efficient, allowing a simultaneous determination of up to nine parameters, including transport and source parameters, such as slope and abundances. Our best-fit model includes both a low energy cut-off and reacceleration, whose values are consistent with those found in diffusion models. A Kolmogorov spectrum for the diffusion slope (δ = 1/3) is excluded. The marginalised probability-density function for δ and α (the slope of the source spectra) are δ ≈ 0.55−0.60 and α ≈ 2.14−2.17, depending on the dataset used and the number of free parameters in the fit. All source-spectrum parameters (slope and abundances) are positively correlated among themselves and with the reacceleration strength, but are negatively correlated with the other propagation parameters. Conclusions. The MCMC is a practical and powerful tool for cosmic-ray physic analyses. It can be used to confirm hypotheses concerning source spectra (e.g., whether αi αj )a nd/or determine whether different datasets are compatible. A forthcoming study will extend our analysis to more physical diffusion models.

Acceleration of High-Energy Cosmic Rays

The transport and acceleration of galactic cosmic rays is discussed with emphasis on the highest energies. The mechanism of diffusive shock acceleration is shown to produce a near-universal spectrum which can account for most observed cosmic rays. A quasi-perpendicular shock in general accelerates particles faster and to higher energies than a comparable quasi-parallel shock. Application of these ideas to supernova blast waves suggests that they can produce galactic cosmic rays to the knee in the spectrum, some 3 10 eV. A similar blast wave, recently identified near the galactic center could accelerate particles to an energy of some 10Z eV or higher (hereinafter Z is the charge in units of the elementary charge). It is possible that the highest-energy cosmic rays could come from the galactic center. If this is the case, there should be an enhancement of the flux of high-energy cosmic rays coming from the galactic center.

Stochastic simulation of cosmic ray modulation including a wavy heliospheric current sheet

We present a quasi‐steady two‐dimensional (axisymmetric) model of the heliospheric transport of galactic cosmic rays. The model is based on stochastic simulation techniques and includes all the modulation mechanisms that cosmic rays experience in the heliosphere: convection, adiabatic cooling, diffusion, and drifts. A special emphasis is given to the cosmic ray transport in the vicinity of the heliospheric current sheet (HCS), and a new method to calculate the wavy current sheet drift is presented. We study cosmic ray modulation in different solar modulation conditions and levels of waviness of the current sheet. We discuss changes in the cosmic ray spectrum and the dominant streaming patterns of cosmic rays in the heliosphere for different solar polarities and HCS tilt angles.

An Overview of the Origin of Galactic Cosmic Rays as Inferred from Observations of Heavy Ion Composition and Spectra

The galactic cosmic rays arriving near Earth, which include both stable and long-lived nuclides from throughout the periodic table, consist of a mix of stellar nucleosynthesis products accelerated by shocks in the interstellar medium (ISM) and fragmentation products made by high-energy collisions during propagation through the ISM. Through the study of the composition and spectra of a variety of elements and isotopes in this diverse sample, models have been developed for the origin, acceleration, and transport of galactic cosmic rays. We present an overview of the current understanding of these topics emphasizing the insights that have been gained through investigations in the charge and energy ranges Z≲30 and E/M≲1 GeV/nuc, and particularly those using data obtained from the Cosmic Ray Isotope Spectrometer on NASA’s Advanced Composition Explorer mission.

Galactic Cosmic Ray modulation based on transport parameters obtained via test particle simulations

Our understanding of Galactic Cosmic Ray (GCR) modulation has advanced greatly in the last years. However, an appropriate knowledge of the components of the diffusion tensor is still lacking. Numerical simulations of test particle trajectories in the layer model of the Heliospheric Magnetic Field allow calculations of local transport coefficients. The layer model makes use of spacecraft data at different heliospheric locations to reproduce transport conditions; there are no special assumptions as to the form of the turbulence. We use these test particle estimated transport coefficients here as parameters for a numerical solution of the GCR transport equation in the case qA > 0. The solutions obtained with this set of parameters reproduce the observed cosmic ray proton spectrum at 1 AU and the radial profiles up to at least 20 AU in the ecliptic plane well.

Evidence for the Nonlinear Transport of Galactic Cosmic Rays

The measured decrease of the abundance ratio of secondary to primary Galactic cosmic-ray nuclei is explained by the increase of the cosmic-ray parallel mean free path with magnetic rigidity λ∥ ∝ R0.6 that results from the weakly nonlinear transport theory of cosmic rays in the turbulent Galactic magnetic fields. Because the ratio of fluctuating to ordered magnetic fields in the Galaxy qL = (δB/B0)2 is large, this nonlinear transport theory has to be favored over the traditional quasi-linear theory that requires very small values qL 1. Our explanation provides an alternative to Galactic transport theories of cosmic rays with significant distributed stochastic acceleration.

Modulation of Cosmic‐Ray Electrons in the Outer Heliosphere

A new cosmic-ray transport model, which investigates the modulation of cosmic-ray electrons in the outer heliospheric regions, including all major modulation mechanisms and the asymmetry of the heliospheric shock structures, is developed. The transport of Galactic cosmic rays into the heliosphere is directly calculated from the local interstellar medium, while for the mutual interaction between the solar wind and the interstellar medium, the three-fluid two-dimensional hydrodynamic Bonn model is used to calculate the geometry of the heliosphere and solar wind flow profiles as input to the transport model. There are considerable differences in the modulation that cosmic rays experience between the heliospheric nose and tail regions, but because of large transport parameters, no modulation of Galactic cosmic rays occurs beyond the heliopause. It is shown that for energetic particles in the range of 1 GeV the termination shock has a significant influence on the modeled Galactic cosmic-ray flux intensities in the equatorial regions in direction to the heliospheric nose, while it is decreasing toward the poles and in the tail. Depending on energy, most of the modulation that the particles experience occurs at radial distances less than the shock radius. It is also shown that the cosmic-ray distribution in the inner heliosphere is not sensitive to the nose-tail asymmetry of the termination shock and heliopause. Finally, the effect of perpendicular transport parameter in the polar region is calculated. It is shown that depending on energy, this parameter influence computed latitudinal gradients, especially at, and to a lesser extend beyond, the termination shock, and in the heliospheric tail.

On the influence of solar activity and the solar magnetic field on the 27-day variation of galactic cosmic rays

The calculated amplitude of the 27-day variation using super neutron monitors data has been compared with changes of the distribution of solar activity on the Sun. Close correlation of the two-peaked pattern of the galactic cosmic rays 27-day variation with changes of the longitudinal asymmetry of the sunspot distribution at the maxima epochs of solar activity has been observed. It is reasonable to assume that drastic changes of the longitudinal asymmetry of solar activity near the solar maximum are reflected in the perturbation of the heliospheric structure, causing the depression of the 27-day modulation. Marked differences between the Sun's magnetic polarity cycles, qA > 0 and qA < 0, were observed in the time profile of the 27-day variation of galactic cosmic rays. The amplitude of the calculated 27-day variation of galactic cosmic rays is not depend on the tilt angles of the heliospheric neutral sheet for both qA > 0 and qA < 0 cycles. The expected amplitude (from theoretical modeling of galactic cosmic rays transport) of the 27-day variation is greater in the qA > 0 than in the qA < 0 solar magnetic cycles being in good agreement with the experimental data for the minima epochs of solar activity.

Galactic cosmic ray transport in the global heliosphere

We present a new axisymmetric model of the heliosphere that includes three principal particle species (plasma with magnetic field, interstellar neutral atoms, and galactic cosmic rays). Unlike previous modulation models, which necessarily included a modulation boundary, we study the entry of the galactic cosmic rays into the heliosphere directly from the local interstellar medium population. An important improvement over previous global heliospheric models is that a kinetic description for the cosmic rays is used instead of the usual fluid approach. We compute the cosmic‐ray diffusion coefficients from quasi‐linear theory, assuming a constant (but different) power spectrum of the fluctuations in both the solar wind and the interstellar medium. Our results show that low‐energy particles are strongly attenuated by the magnetic wall in the inner heliosheath and do not reach the termination shock. The model predicts small cosmic rays gradients in most of the heliosphere, implying little modification to the termination and bow shocks. The exception is the inner heliosheath region, where cosmic‐ray gradients are large and the energetic particles can, consequently, affect the plasma flow. Particle spectra in the inner heliosphere are found to be in agreement with the observations. We also find that the inner heliospheric galactic cosmic‐ray population is not sensitive to the upwind‐downwind asymmetry of the termination shock.

3-D modeling of cosmic-ray transport in the heliosphere: toward solar maximum

Abstract We present preliminary results from new developments in our 3-D time-dependent numerical codes modeling the diffusive transport of galactic and anomalous cosmic rays in the heliosphere, including drift, diffusion, and adiabatic cooling and acceleration. Our existing 3-D codes are modified to achieve a better description of solar maximum conditions. More complex structures of the current sheet are considered, including multiple current sheets during periods of high solar activity. Latitudinal and recurrent time-variations are discussed. The role of the variation of the magnetic field strength is briefly addressed.

Galactic cosmic ray observations at different heliospheric latitudes

The local interstellar spectra (LIS) of galactic cosmic rays at low energies cannot be determined from measurements made near Earth, because of strong solar modulation processes from the heliospheric boundary to the observer. Computations done in the seventies based on Parker's cosmic ray modulation theory taking into account diffusion, convection, adiabatic deceleration, and drift effects in the global heliospheric magnetic field predicted that the cosmic ray proton spectrum, for instance, is only weakly modulated over the poles of the Sun. The data set from the high heliospheric latitude ESA/NASA Ulysses mission, launched in October 1990, is ideally suited to investigate galactic cosmic ray flux variations with heliospheric latitude. Observations from mid 1993 to the end of 1997 during moderate to low solar activity are presented and compared with model predictions. In contrast to what was expected Ulysses did not observe the LIS over the solar poles. However, these observations yield a new understanding of the importance of the processes involved in the transport of galactic cosmic rays, and that the LIS can only be determined by an interstellar mission going far beyond 100 AU.

An overview of the transport of galactic and anomalous cosmic rays in the heliosphere: Theory

The theory for the transport of galactic and anomalous cosmic rays in the heliosphere has been well developed over the last three decades. At present, the dominant physical processes are considered to have been identified and understood. The basic concepts of the theory for cosmic ray transport are reviewed, and the extent to which the theory can account for the observed behavior of the cosmic rays with a consistent choice for the governing parameters is discussed.

Summary talk: The transport of galactic and anomalous cosmic rays in the heliosphere

The field of the transport of galactic and anomalous cosmic rays in the heliosphere shows great vitality, as evidenced by the many excellent papers presented in this symposium. It seems now that the basic transport of energetic charged particles is well understood. The Parker equation is adequate, and what remains is the important and nontrivial task of determining the transport coefficients and the structure of the heliosphere well enough to interpret the cosmic-ray observations. This will permit a better understanding of the physics of cosmic rays, and also lead to more use of cosmic rays as probes of the heliosphere in regions not yet directly explored.

Simulations of galactic and anomalous cosmic ray transport in the heliosphere

The companion paper of Fisk (1999) in this volume convincingly argues that the theory of cosmic ray transport in the heliosphere has probably been complete for almost three decades. The outstanding problem is to find the magnitude, spatial, and energy dependence of the convection, diffusion, drift, adiabatic cooling, and acceleration mechanisms. This problem can be addressed in two complementary ways: the relevant parameters can be calculated from theory, or the observed particle spectra can be used to deduce them. This paper emphasises the second approach, i.e., we review to what extent one is able to fit cosmic ray observations self-consistently with a single set of modulation parameters. For this purpose we mainly concentrate on model fits to the intensity spectra of cosmic ray protons, helium, and oxygen as observed during the solar minimum periods of 1987 and 1997. The main results are that: (1) the scattering parameters deduced from the particle spectra broadly fall in the same range as the newest values derived from theory, but (2) the separate sets of scattering parameters needed to simulate the 1987 and 1997 solar minimum spectra differ so much from one another that it is unlikely that the final solution has been found.

Assessment and requirements of nuclear reaction databases for GCR transport in the atmosphere and structures

The transport properties of galactic cosmic rays (GCR) in the atmosphere, material structures, and human body (self-shielding) are of interest in risk assessment for supersonic and subsonic aircraft and for space travel in low-Earth orbit and on interplanetary missions. Nuclear reactions, such as knockout and fragmentation, present large modifications of particle type and energies of the galactic cosmic rays in penetrating materials. We make an assessment of the current nuclear reaction models and improvements in these model for developing required transport code data bases. A new fragmentation data base (QMSFRG) based on microscopic models is compared to the NUCFRG2 model and implications for shield assessment made using the HZETRN radiation transport code. For deep penetration problems, the build-up of light particles, such as nucleons, light clusters and mesons from nuclear reactions in conjunction with the absorption of the heavy ions, leads to the dominance of the charge Z = 0, 1, and 2 hadrons in the exposures at large penetration depths. Light particles are produced through nuclear or cluster knockout and in evaporation events with characteristically distinct spectra which play unique roles in the build-up of secondary radiation's in shielding. We describe models of light particle production in nucleon and heavy ion induced reactions and make an assessment of the importance of light particle multiplicity and spectral parameters in these exposures.

Relativistic Interaction of22Ne and26Mg in Hydrogen and the Cosmic‐Ray Implications

The isotopic production cross sections for 22Ne projectiles at 377,581, and 894 MeV nucleon-1 and 26Mg projectiles at 371 and 576 MeV nucleon-1 interacting in a liquid hydrogen target have been measured by the Transport Collaboration at the Lawrence Berkeley Laboratory Heavy-Ion Spectrometer System (LBL HISS) facility. These cross sections are compared with those predicted by semi-empirical formulae. The systematics are studied to develop suitable inputs for calculations of galactic cosmic-ray interstellar transport. These calculations are used to unfold the transport effects from available observations of cosmic-ray CNO isotopes to extract the underlying source composition. With these new cross section measurements, the previously reported enhancement of 18O at the cosmic-ray source, which is sensitive to the cross sections for production from 22Ne and 26Mg and the uncertainties in cross section prediction formulae, may be explained. There is no evidence for an enhancement of 18O when these new cross sections are used in a weighted slab propagation calculation.

The influence of intermediate‐scale variations in the heliospheric magnetic field on the transport of galactic cosmic rays

Variations in the heliospheric magnetic field occur on most scales including those lying intermediate between the gyro‐radii of galactic cosmic rays and heliocentric radial distance. It is demonstrated that a correlation exists between these intermediate‐scale variations in the magnetic field and the variations in the cosmic ray distribution function that result from the field variations. This correlation will affect the average transport of galactic cosmic rays by significantly altering the patterns of gradient and curvature drifts in the heliosphere. During the current solar cycle, the altered drift patterns can lead to larger radial gradients of the galactic cosmic rays and significantly smaller latitude gradients than is expected only from drifts in the mean magnetic field.

Solar modulation and nuclear fragmentation effects in galactic cosmic ray transport through shielding

Crews of manned interplanetary missions may accumulate significant radiation exposures from the galactic cosmic ray (GCR) environment in space. Estimates of how these dose levels are affected by the assumed temporal and spatial variations in the composition of the GCR environment, and by the effects of the spacecraft and body self-shielding on the transported radiation fields are presented. In this work, the physical processes through which shielding alters the transported radiation fields are described. We then present estimates of the effects on model calculations of (1) nuclear fragmentation model uncertainties, (2) solar modulation, (3) variations between solar cycles, and (4) proposed changes to the quality factors which relate dose equivalent to absorbed dose.

A program to measure new energetic particle nuclear interaction cross sections

The Transport Collaboration, consisting of researchers from institutions in France, Germany, Italy and the USA, has established a program to make new measurements of nuclear interaction cross sections for heavy projectiles (Z ≥ 2) in targets of liquid H2, He and heavier materials. Such cross sections directly affect calculations of galactic and solar cosmic ray transport through matter and are needed for accurate radiation hazard assessment. To date, the collaboration has obtained data using the LBL Bevalac HISS facility with 20 projectiles from 4He to 58Ni in the energy range 393–910 MeV/nucleon. Preliminary results from the analysis of these data are presented here and compared to other measurements and to cross section prediction formulae.