Propagation Of Galactic Cosmic Rays Research Articles

Dependence of cosmic ray intensity on variation of solar wind velocity measured by the GRAPES-3 experiment for space weather studies

The space weather impacts propagation of galactic cosmic rays (CRs) in the heliosphere as explained by the diffusion-convection mechanism which predicts that the variation in CR intensity should be anticorrelated with changes in solar wind velocity (${\mathrm{V}}_{\mathrm{SW}}$). Several unrelated solar phenomena, including periodic ones such as 27 d solar rotation, annual, 11 yr solar activity, 22 yr solar magnetic cycle etc., and transient ones, for example, Forbush decreases (FDs), and ground level enhancements generally caused by solar flares or coronal mass ejections also affect CR intensity on Earth. These solar phenomena make a quantitative study of dependence of CR intensity on ${\mathrm{V}}_{\mathrm{SW}}$ rather difficult. Here, the high statistics muon data of six years (2000--2005) from the large GRAPES-3 muon telescope have been used to study the correlation between ${\mathrm{V}}_{\mathrm{SW}}$ and CR intensity. Data gathered during these six years were used after minimizing the contribution of various unrelated solar phenomena outlined above. We observed a strong anticorrelation between the variations in ${\mathrm{V}}_{\mathrm{SW}}$ and CR intensity at a significance of $19\ensuremath{\sigma}$.

GALPROP Code for Galactic Cosmic Ray Propagation and Associated Photon Emissions

Recent years are marked with many breakthroughs in astrophysics of cosmic rays (CRs), and more are expected in the nearest future. Their proper interpretation is impossible without a welldeveloped propagation code. The GALPROP project celebrates its 19th anniversary this year. Over these years the project has established itself as a standard self-consistent model for CR propagation in the Galaxy and associated diffuse emissions (radio, microwave, X-rays, g-rays) that is widely used by the astrophysical community. The project stimulated independent studies of the interstellar radiation field, distribution of the interstellar gas (H2, H I, H II), synchrotron emission and the Galactic magnetic field, and a new study of the isotopic production cross sections. These studies provide necessary and unique input datasets for the GALPROP model. The code is optimized and parallelized and accessible as a standalone executable or library that can be linked to other codes enabling many other studies, such as Markov Chain Monte Carlo (MCMC), MultiNest, SuperBayeS, and DarkSUSY. We describe a new updated version of the code that has new capabilities and improved accuracy. As always, the latest release of the code is available through the WebRun, a service to the scientific community enabling easy use of the GALPROP code via web browsers.

NEAR-EARTH COSMIC RAY DECREASES ASSOCIATED WITH REMOTE CORONAL MASS EJECTIONS

Galactic cosmic ray (GCR) flux is modulated by both particle drift patterns and solar wind structures on a range of timescales. Over solar cycles, GCR flux varies as a function of the total open solar magnetic flux and the latitudinal extent of the heliospheric current sheet. Over hours, drops of a few percent in near-Earth GCR flux (Forbush decreases, FDs) are well known to be associated with the near-Earth passage of solar wind structures resulting from corotating interaction regions (CIRs) and transient coronal mass ejections (CMEs). We report on four FDs seen at ground-based neutron monitors which cannot be immediately associated with significant structures in the local solar wind. Similarly, there are significant near-Earth structures which do not produce any corresponding GCR variation. Three of the FDs are during the STEREO era, enabling in situ and remote observations from three well-separated heliospheric locations. Extremely large CMEs passed the STEREO-A spacecraft, which was behind the West limb of the Sun, approximately 2-3 days before each near-Earth FD. Solar wind simulations suggest that the CMEs combined with pre-existing CIRs, enhancing the pre-existing barriers to GCR propagation. Thus these observations provide strong evidence for the modulation of GCR flux by remote solar wind structures.

A stochastic simulation of the propagation of Galactic cosmic rays reflecting the discreteness of cosmic ray sources Age and path length distribution

The path length distribution of Galactic cosmic rays (GCRs) is the fundamental ingredient for modeling the propagation process of GCRs based on the so-called weighted slab method. We try to derive this distribution numerically by taking into account the discreteness in both space and time of occurrences of supernova explosions where GCRs are suspected to be born. We solve numerically the stochastic differential equations equivalent to the Parker diffusion-convection equation which describes the propagation process of GCR in the Galaxy. We assume the three-dimensional diffusion is an isotropic one without any free escape boundaries. We ignore any energy change of GCRs and the existence of the Galactic wind for simplicity. We also assume axisymmetric configurations for the density distributions of the interstellar matter and for the surface density of supernovae. We have calculated age and path length of GCR protons arriving at the solar system with this stochastic method. The obtained age is not the escape time of GCRs from the Galaxy as usually assumed, but the time spent by GCRs during their journey to the solar system from the supernova remnants where they were born. The derived age and path length show a distribution spread in a wide range even for GCR protons arriving at the solar system with the same energy. The distributions show a cut-off at a lower range in age or path length depending on the energy of GCRs. These cut-offs clearly come from the discreteness of occurrence of supernovae. The mean age of GeV particles obtained from the distributions is consistent with the age obtained by direct observation of radioactive secondary nuclei. The energy dependence of the B/C ratio estimated with the path length distribution reproduces reliably the energy dependence of B/C obtained by recent observations in space.

Cosmophysical situation in the epoch of the Cambrian explosion

Based on available published paleodata, cosmophysical environment of the Earth during the Cambrian evolutionary explosion is considered. Some astrophysical data demonstrate that, about 500 Ma, the Sun entered into the Perseus arm with the enhanced density of star population and spent there several dozen million years. According to some meteorite data, the average level of the galactic cosmic ray (GCR) intensity during that period was considerably higher than previously, when the Sun passed the space between the galactic arms. The GCR flux varied from 25 to 135%, as the Solar System consequently crossed other galactic arms. Some correlation of the GCR intensity variations with periods of global warming and cooling has been found. However, there was no unambiguous relation between climatic data and the GCR intensity. Unfortunately, an accuracy of estimation of the GCR intensity through meteorite data varies to within 0.30–1.5, which does not allow making definite conclusions. For more reliable conclusions, additional astrophysical data obtaining is needed, and new approaches to modeling of the GCR propagation, which take into account their characteristic lifetime of ∼10–100 Ma, should be applied. A possibility of impact of Supernova outbursts and superincreases of solar cosmic ray (SCR) flux on climatic changes are discussed. A possible bioeffective role of the geomagnetic field reversals, oscillations of the position of the Sun in the Galaxy, and other unidentified yet factors is also considered.

Spiral arms as cosmic ray source distributions

The Milky Way is a spiral galaxy with (or without) a bar-like central structure. There is evidence that the distribution of suspected cosmic ray sources, such as supernova remnants, are associated with the spiral arm structure of galaxies. It is yet not clearly understood what effect such a cosmic ray source distribution has on the particle transport in our Galaxy. We investigate and measure how the propagation of Galactic cosmic rays is affected by a cosmic ray source distribution associated with spiral arm structures.We use the PICARD code to perform high-resolution 3D simulations of electrons and protons in galactic propagation scenarios that include four-arm and two-arm logarithmic spiral cosmic ray source distributions with and without a central bar structure as well as the spiral arm configuration of the NE2001 model for the distribution of free electrons in the Milky Way. Results of these simulation are compared to an axisymmetric radial source distribution. Also, effects on the cosmic ray flux and spectra due to different positions of the Earth relative to the spiral structure are studied.We find that high energy electrons are strongly confined to their sources and the obtained spectra largely depend on the Earth’s position relative to the spiral arms. Similar finding have been obtained for low energy protons and electrons albeit at smaller magnitude. We find that even fractional contributions of a spiral arm component to the total cosmic ray source distribution influences the spectra on the Earth. This is apparent when compared to an axisymmetric radial source distribution as well as with respect to the Earth’s position relative to the spiral arm structure. We demonstrate that the presence of a Galactic bar manifests itself as an overall excess of low energy electrons at the Earth.Using a spiral arm geometry as a cosmic ray source distributions offers a genuine new quality of modeling and is used to explain features in cosmic ray spectra at the Earth that are else-wise attributed to other propagation effects. We show that realistic cosmic ray propagation scenarios have to acknowledge non-axisymmetric source distributions.

An Alternative Approach to Understanding the Observed Positron Fraction

Space-based observations by PAMELA (Adriani et al., Nature 458, 607, 2009), Fermi-LAT (Ackerman et al., Phys. Rev. Lett. 105, 01103, 2012), and AMS (Aguilar et al., Phys. Rev. Lett. 110, 141102, 2013) have demonstrated that the positron fraction (e+/total-e) increases with increasing energy above about 10 GeV. According to the propagation model for Galactic cosmic rays in widespread use (Moskalenko & Strong, Astrophys. J. 493, 693, 1998), the production of secondary positrons from interaction of cosmic-ray protons and heavier nuclei with the interstellar medium gives a generally falling positron fraction between 10 and 100 GeV, with secondary positrons accounting for only ∼20 % of the observed positron fraction at 100 GeV; so some other physical phenomena have been proposed to explain the data. An alternative approach to interpreting the positron observations is to consider these data as presenting an opportunity for re-examining models of Galactic cosmic-ray propagation. Following release of the PAMELA data, three groups published propagation models (Shaviv, et al., Phys. Rev. Lett. 103, 111302, 2009, Cowsik and Burch, Phys. Rev. D. 82, 023009, 2010, Katz et al., Mon. Not. R. Aston. Soc. 405, 1458 2010) in which the observed positron fraction is explained entirely by secondary positrons produced in the interstellar medium. In May of this year, stimulated by the AMS extension of the positron data to higher energy with excellent statistics, two of those groups presented further development of their calculations (Cowsik et al. 2013, Blum et al. 2013), again concluding that the observed positrons can be understood as secondaries. None of the authors of these five papers was registered for the 33rd International Cosmic Ray Conference (ICRC). Although I am not an author of any of these papers, I have some close familiarity with one of these recent papers, so the conference organizers invited me to bring this alternative approach to the attention of the conference. The present paper is a summary of the material I presented, along with a brief comment about reaction at the conference to this approach.

PICARD: A novel code for the Galactic Cosmic Ray propagation problem

In this manuscript we present a new approach for the numerical solution of the Galactic Cosmic Ray propagation problem. We introduce a method using advanced contemporary numerical algorithms while retaining the general complexity of other established codes. In this paper we present the underlying numerical scheme in conjunction with tests showing the correctness of the scheme. Finally we show the solution of a first example propagation problem using the new code to show its applicability to Galactic Cosmic Ray propagation.

COSMIC RAY PROPAGATION IN GALACTIC TURBULENCE

We revisit propagation of galactic cosmic rays (CRs) in light of recent advances in CR diffusion theory in realistic interstellar turbulence. We use a tested model of turbulence in which it has been shown that fast modes dominate scattering of CRs. As a result, propagation becomes inhomogeneous and environment dependent. By adopting the formalism of the nonlinear theory developed by Yan & Lazarian, we calculate the diffusion of CRs self-consistently from first principles. We assume a two- phase model for the Galaxy to account for different damping mechanisms of the fast modes, and we find that the energy dependence of the diffusion coefficient is mainly affected by medium properties. We show that it gives a correct framework to interpret some of the recent CR puzzles.

PREDICTING THE SUN'S POLAR MAGNETIC FIELDS WITH A SURFACE FLUX TRANSPORT MODEL

The Sun's polar magnetic fields are directly related to solar cycle variability. The strength of the polar fields at the start (minimum) of a cycle determine the subsequent amplitude of that cycle. In addition, the polar field reversals at cycle maximum alter the propagation of galactic cosmic rays throughout the heliosphere in fundamental ways. We describe a surface magnetic flux transport model that advects the magnetic flux emerging in active regions (sunspots) using detailed observations of the near-surface flows that transport the magnetic elements. These flows include the axisymmetric differential rotation and meridional flow and the non-axisymmetric cellular convective flows (supergranules) all of which vary in time in the model as indicated by direct observations. We use this model with data assimilated from full-disk magnetograms to produce full surface maps of the Sun's magnetic field at 15-minute intervals from 1996 May to 2013 July (all of sunspot cycle 23 and the rise to maximum of cycle 24). We tested the predictability of this model using these maps as initial conditions, but with daily sunspot area data used to give the sources of new magnetic flux. We find that the strength of the polar fields at cycle minimum and the polar field reversals at cycle maximum can be reliably predicted up to three years in advance. We include a prediction for the cycle 24 polar field reversal.

Measurement of hydrogen and helium isotopes flux in galactic cosmic rays with the PAMELA experiment

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature, with particular focus on the antimatter component. The detector consists of a permanent magnet spectrometer core to provide rigidity and charge sign information, a Time-of-Flight system for velocity and charge information, a Silicon–Tungsten calorimeter and a Neutron detector for lepton/hadron identification. The velocity and rigidity information allow the identification of different isotopes for Z=1 and Z=2 particles in the energy range 100MeV/n to 1GeV/n. In this work we will present the PAMELA results on the H and He isotope fluxes based on the data collected during the 23rd solar minimum from 2006 to 2007. Such fluxes carry relevant information helpful in constraining parameters in galactic cosmic rays propagation models complementary to those obtained from other secondary to primary measurements such as the boron-to-carbon ratio.

Solar energetic particle drifts in the Parker spiral

Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for solar energetic particles (SEPs). As a result, they have so far been ignored in SEP propagation modeling and data analysis. We examine drift velocities in the Parker spiral within single particle first‐order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy range experience significant gradient and curvature drift. In the scatter‐free case, drift due to magnetic field curvature is present. The magnitude of drift velocity increases by more than an order of magnitude at high heliographic latitudes compared to near the ecliptic; it has a strong dependence on radial distance r from the Sun, reaching a maximum at r∼1 AU at low heliolatitudes and r∼10 AU at high heliolatitudes. Due to the mass over charge dependence of drift velocities, the effect of drift for partially ionized SEP heavy ions is stronger than for protons. Drift is therefore likely to be a considerable source of cross‐field transport for high‐energy SEPs.

THE GALACTIC SPATIAL DISTRIBUTION OF OB ASSOCIATIONS AND THEIR SURROUNDING SUPERNOVA-GENERATED SUPERBUBBLES

Core collapse supernovae of massive (> 8 Mo) stars are formed primarily in OB associations and help blow giant superbubbles, where their collective shocks accelerate most of the Galactic cosmic rays. The spatial distribution of these stars is thus important to our understanding of the propagation of the observed cosmic rays. In order to better model the Galactic cosmic-ray distribution and propagation, we construct s three-dimensional spatial model of the massive star distribution based primarily on the emission of the H II envelopes surrounding the giant superbubbles which are maintaned by the ionizing radiation of the embedded O stars. The Galactic longitudinal distribution of the 205 micron N II rsdistion emitted by these H II envelopes is used to infer the spatial distribution of the superbubbles. We find that the Galactic superbubble distribution is dominated by the contribution of massive star clusters residing in the spiral arms.

Propagation of galactic cosmic rays in the presence of self-generated turbulence

Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. In this paper we present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a pre-existing turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations. The fact that data are explained with no need for artificial breaks in the injection spectrum and/or in the diffusion coefficient as functions of momentum can be interpreted as a strong indication that the phenomenon proposed here is in fact being observed in Nature. We also show that there is very good agreement between the calculated proton spectrum and the cosmic ray spectrum inferred from observations of the gamma ray emission from clouds in the Gould's belt.

PROBING THE PULSAR ORIGIN OF THE ANOMALOUS POSITRON FRACTION WITH AMS-02 AND ATMOSPHERIC CHERENKOV TELESCOPES

Recent observations by PAMELA, Fermi-LAT, and AMS-02 have conclusively indicated a rise in the cosmic-ray positron fraction above 10 GeV, a feature which is impossible to mimic under the paradigm of secondary positron production with self-consistent Galactic cosmic-ray propagation models. A leading explanation for the positron fraction rise is an additional source of electron–positron pairs, for example one or more mature, energetic, and relatively nearby pulsars. We point out that any one of two well-known nearby pulsars, Geminga and Monogem, can satisfactorily provide enough positrons to reproduce AMS-02 observations. A smoking-gun signature of this scenario is an anisotropy in the arrival direction of the cosmic-ray electrons and positrons, which may be detectable by existing, or future, telescopes. The predicted anisotropy level is, at present, consistent with limits from Fermi-LAT and AMS-02. We argue that the large collecting area of atmospheric Cherenkov telescopes (ACTs) makes them optimal tools for detecting such an anisotropy. Specifically, we show that much of the proton and γ-ray background which affects measurements of the cosmic-ray electron–positron spectrum with ACTs may be controlled in the search for anisotropies. We conclude that observations using archival ACT data could already constrain or substantiate the pulsar origin of the positron anomaly, while upcoming instruments (such as the Cherenkov Telescope Array) will provide strong constraints on the source of the rising positron fraction.

TeV cosmic-ray proton and helium spectra in the myriad model

Recent measurements of cosmic ray proton and helium spectra show a hardening above a few hundreds of GeV. This excess is hard to understand in the framework of the conventional models of Galactic cosmic ray production and propagation. We propose here to explain this anomaly by the presence of local sources (myriad model). Cosmic ray propagation is described as a diffusion process taking place inside a two-zone magnetic halo. We calculate the proton and helium fluxes at the Earth between 50 GeV and 100 TeV. Improving over a similar analysis, we consistently derive these fluxes by taking into account both local and remote sources for which a unique injection rate is assumed. We find cosmic ray propagation parameters compatible with B/C measurements and for which the proton and helium spectra remarkably agree with the PAMELA and CREAM measurements over four decades in energy.

Effect of the Heliopause and the Termination shock on the Galactic Cosmic rays distribution in stationary model of heliosphere

Standard model of galactic cosmic rays (GCR) propagation in heliosphere and adjacent interstellar space is considered. Here heliosphere is considered as a two-layered medium consisting of two spherical regions symmetrical about the Sun and adjacent to each other. In the internal region of the heliosphere, which is bounded by termination shock, solar wind has supersonic speed, and in the external region, which is bounded by heliopause, solar wind has subsonic speed. GCR scattering in these regions is happening in different ways and is characterized by corresponding diffusion rates. In the interstellar medium speed of the solar wind is supposed to be equal to zero and scattering became more weak. Here we suppose that there are no any sources of particles on the boundaries between layers. Exact analytical solution of corresponding mathematical problem is not difficult in principle, but is exceedingly bulky, therefore in this work analytic expressions are obtained both for high energy particles (>2500 MeV) and for low energy particles (<1400 MeV) for each region of their propagation. Distribution of the low energy particles confirm to results obtained on the «Voyager» spacecrafts. It is shown that irrespective of the scattering conditions in internal and external parts of the heliosphere density of the low energy particles is continuously growing from the Sun to the heliosphere boundaries.

The 11‐year solar cycle continues during prolonged sunspot minima

Streaming into the solar system at nearly the speed of light, galactic cosmic rays (GCRs) are a high‐energy mix of protons, electrons, and atomic nuclei. As they pass into reach of the outflowing solar wind, the propagation of GCRs is inhibited. Galactic cosmic rays that make it to Earth interact with the atmosphere, creating a shower of heavy isotopes including beryllium‐10. Beryllium‐10 isotope concentrations recorded in ice cores provide a long‐term, high temporal resolution record of galactic cosmic ray flux.

Effect of the heliosheath and standing termination shock on galactic cosmic ray propagation in a stationary heliosphere model

We consider a stationary model of the propagation of galactic cosmic rays (GCR) in the heliosphere and adjacent interstellar space. The heliosphere is assumed to be a two-layer medium consisting of two adjacent regions that are spherically symmetric relative to the sun. The solar wind velocity is supersonic in the inner heliosphere bounded by the standing termination shock, and this velocity is subsonic in the outer heliosphere bounded by the heliosheath. The GCR scattering in these regions is due to different factors characterized by relevant diffusion coefficients. The solar wind velocity is assumed to be zero in the interstellar medium, where the scattering becomes weaker. No particle sources are presumed to exist at the boundaries between the layers. An exact analytical solution of the corresponding mathematical problem can be obtained without essential difficulties, although it is extremely cumbersome. Analytical expressions for the GCR spectra of particles with very high energies (>2500 MeV) and very low energies (<1400 MeV) are obtained for each region of particle propagation. The low-energy particle distribution corresponds to the data obtained by the Voyager spacecraft. It is shown that the low-energy particle density continuously increases from the sun toward the heliospheric boundary, regardless of the scattering mode in the inner and outer parts of the heliosphere.

Direct measurements of galactic cosmic-ray energy spectra and elemental composition

The description of the Advanced Thin Ionization Calorimeter (ATIC) experiment and its main results are presented. The ATIC experiment is an important step in Galactic cosmic-ray investigations that use the direct methods that were first carried out in the PROTON experiment in the 1960s. We consider the contribution of the ATIC experiment to the understanding of the Galactic cosmic ray origin and propagation.