Distribution Of Galactic Cosmic Rays Research Articles

Cosmic-Ray Transport near the Sun

Abstract The strongly diverging magnetic field lines in the very inner heliosphere, through the associated magnetic focusing/mirroring forces, can, potentially, lead to highly anisotropic galactic cosmic-ray distributions close to the Sun. Using a simplified analytical approach, validated by numerical simulations, we study the behavior of the galactic cosmic-ray distribution in this newly explored region of the heliosphere and find that significant anisotropies can be expected inside 0.2 au.

Can the TeV gamma-ray sky probe the galactic cosmic ray distribution?

We evaluate the diffuse gamma-ray flux at TeV energies produced by hadronic interactions of cosmic rays with the gas contained in the galactic disk. We consider different assumptions for the cosmic ray distribution, including the recently emerged possibility of a harder cosmic ray spectrum in the inner Galaxy. We show that observational data provided by Argo-YBJ, HESS, HAWC and Milagro, can already discriminate among different hyphoteses. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.

Probing galactic cosmic ray distribution with TeV gamma-ray sky

The distribution of cosmic rays in the Galaxy is still uncertain and this affects the expectations for the diffuse gamma-ray emission produced by hadronic interactions of cosmic rays with the interstellar gas. We evaluate the diffuse gamma-ray flux at TeV energies by considering different assumptions for the cosmic ray distribution, including the recently emerged possibility of a decreasing cosmic ray spectral index in the inner Galaxy. The diffuse emission from the galactic central region (i.e. in the longitude range | l | ≤ 60̂) changes in the different scenarios by a relatively large factor and can be probed by TeV scale gamma-ray observations. By comparing the total flux produced by diffuse emission and point-like and extended sources resolved by HESS with the gamma-ray flux observed by Argo-YBJ, HESS, HAWC and Milagro, we show that experimental data can already discriminate among different hyphoteses for cosmic ray distribution. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.

Investigation of mechanisms of formation of the second spherical harmonics of the galactic cosmic ray angular distribution

Mechanisms of formation of a second spherical harmonic of an angular distribution of galactic cosmic rays in the heliosphere are investigated. On the basis of data of Nagoya multidirectional muon telescope using a method of tensor anisotropy of cosmic rays time courses of symmetric diurnal, antisymmetric diurnal and semidiurnal variations of cosmic rays were obtained for the period 1971-2017. An abrupt offset of a phase of the semidiurnal variation to the earlier time during a positive polarity of the general magnetic field of the Sun, which closely correlates with the temporal change of a phase of the symmetric diurnal variations. It was shown the existence of annual variations of the tensor anisotropy components, which can be satisfactorely described by a single modulation factor - a screening of cosmic rays by the sector-structured interplanetary magnetic field. Moreover, the theory indicates on the existense of a stable north-south asymmetry of the interplanetary magnetic field - shift of the neutral current sheet to the south of solar equator.

New look on the origin of cosmic rays

From the analysis of the flux of high energy particles, E > 3 · 10 18 eV, it is shown that the distribution of the power density of extragalactic rays over energy is of the power law, q ( E ) ∝ E −2.7 , with the same index of 2.7 that has the distribution of Galactic cosmic rays before the so called ‘knee', E 15 eV. However, the average power of extragalactic sources, which is of e ≃ 10 43 erg s −1 , exceeds by at least two orders the power emitted by the Galaxy in cosmic rays, assuming that the density of galaxies is estimated as N g ≃ 1 Mpc −3 . Considering that such power can be provided by relativistic jets from active galactic nuclei with the power e ≃ 10 45 − 10 46 erg s −1 , we estimate the density of extragalactic sources of cosmic rays as N g ≃ 10 −2 − 10 −3 Mpc −3 . Assuming the same nature of Galactic and extragalactic rays, we conclude that the Galactic rays were produced by a relativistic jet emitted from the Galactic center during the period of its activity in the past. The remnants of a bipolar jet are now observed in the form of bubbles of relativistic gas above and below the Galactic plane. The break, observed in the spectrum of Galactic rays (‘knee’), is explained by fast escape of energetic particles, E > 3 · 10 15 eV, from the Galaxy because of the dependence of the coefficient of diffusion of cosmic rays on energy, D ∝ E 0.7 . The obtained index of the density distribution of particles over energy, N ( E )∝ E −2.7−0.7/2 = E −3.05 , for E > 3 · 10 15 eV agrees well with the observed one, N ( E )∝ E −3.1 . The estimated time of the termination of the jet in the Galaxy is 4.2 · 10 4 years ago.

The effect of solar wind high-speed streams on the galactic cosmic rays intensity

The effect of high-speed recurrent solar wind streams from coronal holes on the galactic cosmic rays intensity is investigated. The distribution of galactic cosmic rays for different solar cycles is considered based on the data of the world network of neutron monitors. Within the inhomogeneous model, which includes a homogeneous background and regions of high-speed streams (HSS’s), the transport equation has been solved and the effect of HSS’s on the spatial distribution of galactic cosmic rays is estimated. It is shown that theoretical calculations are agreed with the experimental results obtained for 2000–2014 under different assumptions about the mean free path of cosmic rays in the corresponding period of HSS’s.

On the radial distribution of Galactic cosmic rays

Abstract The spectrum and morphology of the diffuse Galactic γ-ray emission carries valuable information on cosmic ray (CR) propagation. Recent results obtained by analyzing Fermi-LAT data accumulated over 7 yr of observation show a substantial variation of the CR spectrum as a function of the distance from the Galactic Centre. The spatial distribution of the CR density in the outer Galaxy appears to be weakly dependent upon the galactocentric distance, as found in previous studies as well, while the density in the central region of the Galaxy was found to exceed the value measured in the outer Galaxy. At the same time, Fermi-LAT data suggest a gradual spectral softening while moving outwards from the centre of the Galaxy to its outskirts. These findings represent a challenge for standard calculations of CR propagation based on assuming a uniform diffusion coefficient within the Galactic volume. Here, we present a model of non-linear CR propagation in which transport is due to particle scattering and advection off self-generated turbulence. We find that for a realistic distribution of CR sources following the spatial distribution of supernova remnants and the space dependence of the magnetic field on galactocentric distance, both the spatial profile of CR density and the spectral softening can easily be accounted for.

Studying the distribution and the origin of Galactic cosmic rays by meter wavelength radio observation

The origin, propagation and distribution of Galactic cosmic rays is one of the important fields of cosmic ray study even after more than 100 years since its discovery. Although great progress has been made, It is still far from the final answer. In recent years, joint observations of radio, optical, X-ray and γ -ray play an important role in understanding the Galactic cosmic rays. These observations provide a lot of evidences supporting that Galactic cosmic rays are mainly accelerated in the shock of supernova remnant through the mechanism of diffusive shock acceleration. Our knowledge about the distribu-tion and composition of Galactic cosmic rays is also improved by analyzing the high sensitivity diffuse γ -ray data of Fermi . Meterwave radio observation may promote the studies of Galactic cosmic rays distribution, propagation (absorption obser-vation of HII and planetary nebulae, high sensitivity and resolution meter wavelength sky survey) and origin (observation of supernova remnants shock region, scintillation and scattering observation of extragalactic point sources and pulsars, searching the lower energy counterparts of TeV γ -ray sources).

On the coronal effect on the distribution of galactic cosmic rays in the absence of their interaction with the interstellar medium

Two problems of the stationary galactic cosmic ray (GCR) modulation in the closed heliospheric model are considered. It is assumed that the heliosphere is a spherically symmetric medium, which is limited by the heliopause, and GCRs propagate from it to the region spherically symmetric with respect to the Sun (corona). The GCR scattering beyond the coronal region is characterized by the constant diffusion coefficient. The solar wind velocity is also constant in this region. In the first problem GCRs are absorbed at the coronal boundary due to the interaction with the coronal material. In the second problem, GCRs are elastically scattered by coronal magnetic fields. As a result of solving these problems in the paper, it is shown that the coronal effect on the GCR modulation occurs only in a restricted region near the Sun.

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.

GALACTIC AND EXTRAGALACTIC SUPERNOVA REMNANTS AS SITES OF PARTICLE ACCELERATION

Supernova remnants, owing to their strong shock waves, are likely sources of Galactic cosmic rays. Studies of supernova remnants in X-rays and gamma rays provide us with new insights into the acceleration of particles to high energies. This paper reviews the basic physics of supernova remnant shocks and associated particle acceleration and radiation processes. In addition, the study of supernova remnant populations in nearby galaxies and the implications for Galactic cosmic ray distribution are discussed.

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.

Radial distribution of galactic cosmic rays at solar maximum

In this paper we analyze the spatial distribution of galactic cosmic rays during periods of maximum solar activity of the cycles 21, 22 and 23. We have used a two dimensional model to solve the cosmic ray transport equation. This model includes all relevant physical processes: diffusion, convection, drift and shock effects on cosmic ray propagation inside the heliosphere. We focused on the study of the radial distribution of galactic cosmic rays, and compare our results with the spacecraft observations for two energies (175MeV H and 265MeV/n He). Although the radial intensities of galactic cosmic rays can be explained qualitatively with all three local interstellar spectra (LISs) used in this work, we applied a reduced chi-squared analysis to investigate the best LIS that could fit the data.

OBSERVATION OF ANISOTROPY IN THE GALACTIC COSMIC-RAY ARRIVAL DIRECTIONS AT 400 TeV WITH ICECUBE

In this paper we report the first observation in the Southern hemisphere of an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This measurement was performed using cosmic ray induced muons recorded by the partially deployed IceCube observatory between May 2009 and May 2010. The data include a total of 33$\times 10^{9}$ muon events with a median angular resolution of $\sim3^{\circ}$ degrees. A sky map of the relative intensity in arrival direction over the Southern celestial sky is presented for cosmic ray median energies of 20 and 400 TeV. The same large-scale anisotropy observed at median energies around 20 TeV is not present at 400 TeV. Instead, the high energy skymap shows a different anisotropy structure including a deficit with a post-trial significance of -6.3$\sigma$. This anisotropy reveals a new feature of the Galactic cosmic ray distribution, which must be incorporated into theories of the origin and propagation of cosmic rays.

Anomalous and Galactic Cosmic Rays at 1 AU During the Cycle 23/24 Solar Minimum

Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum generally track galactic cosmic ray (GCR) intensities such as those measured by neutron monitors, albeit with differences between solar polarity cycles. The unusual cycle 23/24 solar minimum was long-lasting with very low sunspot numbers and significantly reduced interplanetary magnetic field strength and solar wind dynamic pressure and turbulence, but also featured a heliospheric current sheet tilt that remained high for an extended period. Peak ACR intensities did not recover to the maximum values reached during the last two A>0 solar minima and just barely reached the last A<0 levels. However, GCR intensities in 2009 (neutron monitor rates and also at ∼200 MeV/nucleon) were the highest recorded during the last 50 years, indicating their intensities were not as heavily modulated during their transport from the outer heliosphere. This unexpected difference in the behavior of ACRs and GCRs remains unexplained, but suggests that either the ACR source intensity may have weakened since the last A<0 epoch, or perhaps that ACR intensities at 1 AU in the ecliptic may be more sensitive than GCRs to the higher tilt angle. This seems plausible if the ACR source intensity is greater at low latitudes during A<0 cycles, while the GCR distribution at the heliospheric boundary is more uniform in latitude. Shortly after an abrupt increase in the current sheet tilt angle in late 2009, both ACR and GCR intensities showed dramatic decreases, marking the end of solar minimum modulation conditions for this cycle.

Galactic cosmic ray distribution in the simplest model of termination shock near the heliospheric boundaries

The termination shock at the heliospheric boundary is simulated in terms of a two-layer turbulent medium for which the average radial component of solar wind velocity is nonzero inside the heliosphere and zero for external magnetic inhomogeneities. Galactic cosmic rays (GCRs) are scattered more strongly in the solar wind than in the interstellar medium. A boundary value problem for density is defined to describe GCR propagation in the given two-layer medium. The exact analytical solution of it is derived. The phase density and GCR fluxes in the whole range of the particle energies, as well as the degree of anisotropy of high-energy GCRs, are determined. The qualitative agreement of theoretical calculations and observed GCR distributions is obtained. In particular, in the region near the termination shock, an increase in the high-energy particle density and a decrease in the low-energy particle density are observed.

Radial intensity gradients of galactic cosmic rays in the heliosphere at solar maximum: 1D no-shock simulation

&#x0D; &#x0D; &#x0D; We study the spatial distribution of galactic cosmic rays in the heliosphere at solar maximum of cycles 21, 22 and 23, using a one-dimensional no-shock model of the cosmic ray transport equation. We investigate the radial intensity gradients from 1 AU to the distant heliosphere and interpret the data from IMP8, Voyagers 1 and 2, Pioneer 10 and balloon experiment BESS. We consider three physical processes that affect cosmic radiation: diffusion, convection and adiabatic energy loss. Our analysis indicates that adiabatic energy may play an impor- tant role in the radial distribution of galactic cosmic rays in the inner heliosphere. In the outer region diffusion and convection are the dominant processes.&#x0D; &#x0D; &#x0D;

Latitudinal Gradients of Galactic Cosmic Rays during the 2007 Solar Minimum

Ulysses, launched in 1990 October in the maximum phase of solar cycle 22, completed its third out-of-ecliptic orbit in 2008 February. This provides a unique opportunity to study the propagation of cosmic rays over a wide range of heliographic latitudes during different levels of solar activity and different polarities in the inner heliosphere. Comparison of the first and second fast latitude scans from 1994 to 1995 and from 2000 to 2001 confirmed the expectation of positive latitudinal gradients at solar minimum versus an isotropic Galactic cosmic ray distribution at solar maximum. During the second scan in mid-2000, the solar magnetic field reversed its global polarity. From 2007 to 2008, Ulysses made its third fast latitude scan during the declining phase of solar cycle 23. Therefore, the solar activity is comparable in 2007-2008 to that from 1994 to 1995, but the magnetic polarity is opposite. Thus, one would expect to compare positive with negative latitudinal gradients during these two periods for protons and electrons, respectively. In contrast, our analysis of data from the Kiel Electron Telescope aboard Ulysses results in no significant latitudinal gradients for protons. However, the electrons show, as expected, a positive latitudinal gradient of ~0.2% per degree. Although our result is surprising, the nearly isotropic distribution of protons in 2007-2008 is consistent with an isotropic distribution of electrons from 1994 to 1995.

Study of the charge distribution of galactic cosmic rays and search for superheavy nuclei traces in olivine crystals from meteorites

The search for and identification of energetic nuclei of superheavy elements of cosmic rays in olivine crystals frommeteorites, currently performedwithin the Olympia project [1], are based on measurements of dynamic and geometrical parameters of tracks, i.e., chemically etchable regions of the traces of slowing down of these nuclei before their stop, using the fully automated PAVIKOM measuring system [2].

Formation of higher harmonics of the galactic cosmic-ray distribution function

We derive equations for the multipole moments of the distribution function of Galactic cosmic rays with energies 1–20 TeV that experience random scattering by turbulence with a power-law spectrum. We take into account the irregularity of the local interstellar medium (LISM) in the neighborhood of the solar system due to the presence of interstellar clouds, the interstellar wind flow around the heliomagnetosphere, and preceding supernova explosions in the local superbubble. The amplitudes of the second and third harmonics of the cosmic-ray distribution function are expressed in terms of the amplitude of the first harmonic without assuming them to be small compared to the first harmonic. Reconciling their values in magnitude and phase with the observed values requires a significant LISM irregularity, which is consistent with other observational data on the LISM structure. Our model is consistent with the assumption that supernova remnants in the Galactic disk located at distances from the Solar system much larger than the particle transport mean free path are the sources of the particles under consideration.