Cosmic Ray Anisotropy Research Articles

Cosmic Rays During the Geomagnetic Disturbance in January 2015

According to the data of ground-based measurements of cosmic rays (CRs) from the world network of stations, the behavior of CR intensity during the geomagnetic disturbance in January 2015 has been studied using the spectrographic global survey method. The spectra of CR variations and spectrum indexes of these variations are presented, when approximating the spectrum by the power function of the particle rigidity in the rigidity range from 10 to 50 GV, as well as pitch-angular anisotropy of CRs. It is shown that the spectrum indexes of CR variations at its approximation by the power function of the particle rigidity in the phase of maximum modulation are greater than in the phases of decay and recovery of CR intensity.

Investigation of Ultrahigh-Energy Cosmic-Ray Anisotropy Using the Data on Muon Bundles Recorded with the DECOR Coordinate-Tracking Detector

Results of investigation of the high-energy cosmic-ray flux anisotropy using data on muon bundles are presented. The events were recorded in the period of 2013–2017 with the DECOR coordinate-tracking detector, which is a part of the unique scientific setup of the Experimental complex NEVOD. Methods for experimental data processing and taking atmospheric effects into account are described. Results of the analysis of two types of muon bundle events, which correspond to different ranges of primary particle energies (E > 1015 eV and E > 1016 eV) are discussed. Upper limits of the anisotropy amplitude for the two types of events are obtained.

Revisiting the Spatially Dependent Propagation Model with the Latest Observations of Cosmic-Ray Nuclei

Abstract Recently the AMS-02 collaboration published detections of light cosmic-ray nuclei, including lithium, beryllium, boron, carbon, and oxygen. Combined with the released energy spectra of proton and helium, both primary and secondary spectra have a prominent hardening above ∼200 GV. In particular, the spectral variation of secondary cosmic rays is greater than the primary ones. One of the plausible interpretations for the above anomalies is the spatial-dependent diffusion model. It has successfully described various observational phenomena, e.g., hardening of primary nuclei, diffuse gamma-ray distribution and cosmic-ray anisotropy, etc. In this work, we apply the spatial-dependent propagation model to the latest observations, including both primary and secondary nuclei. Apart from the primary components, the spectra of secondary nuclei can be properly reproduced as well, especially the upturn above 200 GV. We also calculate the ratios of both secondary-to-primary and secondary-to-secondary. We find that except for the Be/B ratio, the computations of the spatial-dependent propagation model are in good agreement with the current data.

The Role of Cosmic-ray Transport in Shaping the Simulated Circumgalactic Medium

Abstract The majority of galactic baryons resides outside of the galactic disk in the diffuse gas known as the circumgalactic medium (CGM). While state-of-the art simulations excel at reproducing galactic disk properties, many of them struggle to drive strong galactic winds or to match the observed ionization structure of the CGM using only thermal supernova feedback. To remedy this, recent studies have invoked nonthermal cosmic ray (CR) stellar feedback prescriptions. However, numerical schemes of CR transport are still poorly constrained. We explore how the choice of CR transport affects the multiphase structure of the simulated CGM. We implement anisotropic CR physics in the astrophysical simulation code Enzo and simulate a suite of isolated disk galaxies with varying prescriptions for CR transport: isotropic diffusion, anisotropic diffusion, and streaming. We find that all three transport mechanisms result in strong, metal-rich outflows but differ in the temperature and ionization structure of their CGM. Isotropic diffusion results in a spatially uniform, warm CGM that underpredicts the column densities of low ions. Anisotropic diffusion develops a reservoir of cool gas that extends farther from the galactic center, but disperses rapidly with distance. CR streaming projects cool gas out to radii of 200 kpc, supporting a truly multiphase medium. In addition, we find that streaming is less sensitive to changes in constant parameter values like the CR injection fraction, transport velocity, and resolution than diffusion. We conclude that CR streaming is a more robust implementation of CR transport and motivates the need for detailed parameter studies of CR transport.

Large-scale Cosmic-Ray Anisotropies above 4 EeV Measured by the Pierre Auger Observatory

Abstract We present a detailed study of the large-scale anisotropies of cosmic rays with energies above 4 EeV measured using the Pierre Auger Observatory. For the energy bins [4, 8] EeV and E ≥ 8 EeV, the most significant signal is a dipolar modulation in R.A. at energies above 8 EeV, as previously reported. In this paper we further scrutinize the highest-energy bin by splitting it into three energy ranges. We find that the amplitude of the dipole increases with energy above 4 EeV. The growth can be fitted with a power law with index β = 0.79 ± 0.19. The directions of the dipoles are consistent with an extragalactic origin of these anisotropies at all the energies considered. Additionally, we have estimated the quadrupolar components of the anisotropy: they are not statistically significant. We discuss the results in the context of the predictions from different models for the distribution of ultrahigh-energy sources and cosmic magnetic fields.

Spectra and Anisotropy of Cosmic Rays During the First GLE Event of Solar Cycle 24

By employing the method of spectrographic global survey, the spectra of protons and anisotropy of cosmic rays during the firstGround-Level Enhancement (GLE) of the intensity of cosmic rays in the solar cycle 24 onMay 17, 2012, a GLE71 event, are studied on the basis of data from ground-based observations of cosmic rays by means of the worldwide network of stations and data from spacecrafts. The rigidity spectra of cosmic rays and relative variations in the intensity of cosmic rays with rigidity 2 GV in the solarecliptic geocentric coordinate system are presented within individual periods of the event under study. It is shown that, in GLE71 periods, protons were accelerated to rigidities in the range of R ~ 2.3−2.5 GV and that the respective differential rigidity spectra of solar cosmic rays are not described by a power-law or an exponential function of the particle rigidity. During the event being considered, the Earth was in the loop-shaped structure of the interplanetary magnetic field.

Puzzles of the cosmic ray anisotropy

We discuss three of the known puzzles of the cosmic ray anisotropy in the PeV and sub-PeV energy region. They are (1) the so called inverse anisotropy, (2) the irregularity in the energy dependence of the amplitude and phase of the first harmonic and (3) the contribution of the single source.

Observation of Anisotropy of TeV Cosmic Rays with Two Years of HAWC

Abstract After two years of operation, the High-Altitude Water Cherenkov (HAWC) Observatory has analyzed the TeV cosmic-ray sky over an energy range between 2.0 and 72.8 TeV. Like other detectors in the northern and southern hemispheres, HAWC observes an energy-dependent anisotropy in the arrival direction distribution of cosmic rays. This anisotropy is dominated by a dipole moment with phase in R.A. α ≈ 40° and amplitude that slowly rises in relative intensity from 8 × 10−4 at 2 TeV to 14 × 10−4 around 30 TeV, above which the dipole decreases in strength. A significant large-scale (>60° in angular extent) signal is also observed in the quadrupole and octupole moments, and significant small-scale features are also present, with locations and shapes consistent with previous observations. Compared to previous measurements in this energy range, the HAWC cosmic-ray sky maps improve on the energy resolution and fit precision of the anisotropy. These data can be used in an effort to better constrain local cosmic-ray accelerators and the intervening magnetic fields.

A Few Selected Topics in Extreme Astrophysical Phenomena: Gamma-ray Burst as a Source of Multi-messenger Astrophysics and Cosmic Particles as a Would-be Messenger

These days of astrophysics are full of exciting discoveries. Gravitational waves are directly found for the first time by Laser Interferometer Gravitational-wave Observatory. Merging of compact astrophysical objects (neutron star and neutron star) is detected via multi-messengers of gravitational wave and all wavelengths of photons for the first time by worldwide observatories on ground and in space. Interesting unknowns of the gamma-ray sky are introduced by Fermi, and many details of gamma-ray bursts (GRBs) become unveiled by the Swift space mission and ground based observatories. Astrophysical neutrinos with the highest energy are detected by the IceCube telescope. Hot and Warm sports, anisotropy of ultra-high energy cosmic rays, are found by the Telescope Array and Pierre Auger observatories, respectively. The spectral break in cosmic rays around 200 GeV is discovered and verified for all elements of cosmic rays, which requires a completely different paradigm in cosmic ray propagation. Combining all of those information allows us to learn the structure and evolution of the universe including thermal and non-thermal astrophysical sources, their spatial distributions, time evolutions and properties. Here we overview a few topics particularly emphasizing GRBs, as sources of multi-messenger astrophysics, in connection to GWs, neutrinos, and cosmic rays. Finally we introduce our space experiments on GRBs and cosmic rays for those physics targets.

Spatial-Energy Characteristics of Cosmic Rays and Parameters of Magnetospheric Current Systems in March and June 2015

The data from terrestrial observations of cosmic rays at the global network of stations by the method of spectrographic global survey were used to analyze two Forbush decreases during the geomagnetic storms in March and June 2015. The spectra of cosmic ray variations, pitch angle anisotropy of cosmic rays at different phases of Forbush decrease development, and the changes in the planetary system of geomagnetic cutoff rigidities are presented. It is shown that, during the approximation of the spectra of variations by the power function of particle rigidity in the interval of 10–50 GV, the spectrum index is softer at the maximum modulation phase than during the phases of cosmic ray intensity decline and recovery. In the axisymmetric model of the bounded magnetosphere of the Earth, which takes into account the currents at the magnetopause and the ring current, the distance to the subsolar point and the radius of the ring current, as well as the contribution of the ring current to the changes in geomagnetic cutoff rigidity and to the Dst index during the studied events, are determined.

Searching for All-scale Anisotropies in the Arrival Directions of Cosmic Rays above the Ankle

Abstract The Pierre Auger Observatory has recently reported the detection of a dipole anisotropy in the arrival directions of cosmic rays above 8 EeV with a post-trial significance of more than 5.2σ. This observation has profound consequences for the distribution and composition of candidate sources of cosmic rays above the ankle (3–5 EeV). In this paper we search for the presence of anisotropies on all angular scales in public Auger data. The analysis follows a likelihood-based reconstruction method that automatically accounts for variations in the observatory’s angular acceptance and background rate. Our best-fit dipole anisotropy in the equatorial plane has an amplitude of 5.3% ± 1.3% and R.A. angle of 103° ± 15°, consistent with the results of the Pierre Auger Collaboration. We do not find evidence for the presence of medium- or small-scale anisotropies. The method outlined in this paper is well-suited for future analyses of cosmic ray anisotropies below the ankle, where cosmic ray detection in surface arrays is not fully efficient and dominated by systematic uncertainties.

Cosmic-Ray Short Burst Observed with the Global Muon Detector Network (GMDN) on 2015 June 22

Abstract We analyze the short cosmic-ray intensity increase (“cosmic-ray burst”: CRB) on 2015 June 22 utilizing a global network of muon detectors and derive the global anisotropy of cosmic-ray intensity and the density (i.e., the omnidirectional intensity) with 10 minute time resolution. We find that the CRB was caused by a local density maximum and an enhanced anisotropy of cosmic rays, both of which appeared in association with Earth’s crossing of the heliospheric current sheet (HCS). This enhanced anisotropy was normal to the HCS and consistent with a diamagnetic drift arising from the spatial gradient of cosmic-ray density, which indicates that cosmic rays were drifting along the HCS from the north of Earth. We also find a significant anisotropy along the HCS, lasting a few hours after the HCS crossing, indicating that cosmic rays penetrated into the inner heliosphere along the HCS. Based on the latest geomagnetic field model, we quantitatively evaluate the reduction of the geomagnetic cutoff rigidity and the variation of the asymptotic viewing direction of cosmic rays due to a major geomagnetic storm that occurred during the CRB and conclude that the CRB is not caused by the geomagnetic storm, but by a rapid change in the cosmic-ray anisotropy and density outside the magnetosphere.

Galactic Cosmic-Ray Anisotropy in the Northern Hemisphere from the ARGO-YBJ Experiment during 2008–2012

Abstract This paper reports on the observation of the sidereal large-scale anisotropy of cosmic rays using data collected by the ARGO-YBJ experiment over 5 years (2008–2012). This analysis extends previous work limited to the period from 2008 January to 2009 December, near the minimum of solar activity between cycles 23 and 24. With the new data sample, the period of solar cycle 24 from near minimum to maximum is investigated. A new method is used to improve the energy reconstruction, allowing us to cover a much wider energy range, from 4 to 520 TeV. Below 100 TeV, the anisotropy is dominated by two wide regions, the so-called “tail-in” and “loss-cone” features. At higher energies, a dramatic change of the morphology is confirmed. The yearly time dependence of the anisotropy is investigated. Finally, no noticeable variation of cosmic-ray anisotropy with solar activity is observed for a median energy of 7 TeV.

Investigation of the Cosmic Rays Flux Anisotropy at High Energies (1015–1017 eV) by Means of Muon Bundles Registered with the Coordinate-Tracking Detector DECOR

First results of the analysis of cosmic ray anisotropy from muon bundles recorded in the coordinate tracking detector DECOR of the Experimental Complex NEVOD during the 2013–2017 measurement series are discussed. Two event samples differing in the multiplicity of muons, selection conditions, and ranges of the zenith and azimuth angles are analyzed. Estimates of the anisotropy parameters of the flux of primary cosmic rays with energies of 1015–1017 eV are presented.

Where do the ultra-high energy cosmic rays come from?

Cosmic rays are the only sample of matter coming from outer space. They carry rich information about particle physics, high-energy astrophysics, composition and evolution of the Universe. The highest energy of cosmic rays detected so far is about 3×10 20 eV. How are the cosmic rays accelerated?What kind of astronomic sources do the cosmic rays originate from?Are the fundamental physics laws still valid in such high-energy scales? are all important scientific problems waiting to be addressed. To answer these questions, various space and ground experiments have been carried out to explore the nature of cosmic rays in a multi-messenger approach. The past few decades have witnessed great achievements in high-energy cosmic ray, gamma ray, neutrino observations, and detection of gravitational wave. (1) The cosmic ray spectra, composition, and anisotropy have been measured with unprecedented precision leading to better understanding of cosmic ray acceleration and transport and stricter constraints on properties of potential candidates of dark matter particles, and the dipole anisotropy of ultrahigh energy cosmic rays indicates that they have an extragalactic origin. (2) Space borne experiments have discovered more than 3000 sources in the GeV range, and the ground based experiments have uncovered more than 200 sources at TeV energies. Most of these sources are high-energy electron accelerators, and a few of them are identified as the accelerators of cosmic ray nuclei. Diffuse gamma-ray emission associated with galactic disks and jets and/or outbursts of Active Galactic Nuclei is also measured with better spatial and energy resolutions, which can be used to study cosmic ray transport in the interstellar/galactic/cluster medium. (3) With the IceCube experiment, about 100 high-energy neutrino events have been recorded and their isotropic distribution suggests an extragalactic origin. (4) The first gravitational wave event with simultaneous multi-wavelength observations has been detected opening the epoch of gravitational wave astronomy. Strong gravitational wave events represent the most catastrophic energy release in the Universe and can be important cosmic ray sources. These new results established the foundation to address the origin of cosmic rays and to develop theories of particle acceleration and transport. The study of cosmic ray transport has gone beyond the diffusion approximation to study the effect of magnetic field fluctuations on small scale anisotropy of cosmic rays and comprehensive numerical modelings of cosmic ray transport in the Galaxy are advanced with multi-wavelength observations. Diffusive particle acceleration by strong shocks of supernova remnants (SNRs) is also advanced based on multi-wavelength observations and cosmic ray measurements. The scenario of SNR origin of Galactic cosmic rays is being quantified with testable predictions. Although there are still uncertainties in amplification of magnetic field by cosmic rays and cosmic ray scattering by turbulent plasmas, sophisticated numerical codes are being developed to address these issues. Successful construction and operation of new generation cosmic ray and gamma-ray experiments will open a new chapter of the astro-particle physics study.

An Upper Limit on the Strength of the Extragalactic Magnetic Field from Ultra-high-energy Cosmic-Ray Anisotropy

Abstract If ultra-high-energy cosmic rays originate from extragalactic sources, the offsets of their arrival directions from these sources imply an upper limit on the strength of the extragalactic magnetic field (EGMF). The Pierre Auger Collaboration has recently reported that anisotropy in the arrival directions of cosmic rays is correlated with several types of extragalactic objects. If these cosmic rays originate from these objects, they imply a limit on the EGMF strength of B < 0.7– G for coherence lengths λ B < 100 Mpc and B < 0.7–2.2 × 10−10 G at larger scales. This is comparable to existing upper limits at λ B = 1 Mpc, and improves on them by a factor of 4–12 at larger scales. The principal source of uncertainty in our results is the unknown cosmic-ray composition.

Global Survey Method for the World Network of Neutron Monitors

One of the variants of the global survey method developed and used for many years at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences is described. Data from the world network of neutron monitors for every hour from July 1957 to the present has been processed by this method. A consistent continuous series of hourly characteristics of variation of the density and vector anisotropy of cosmic rays with a rigidity of 10 GV is obtained. A database of Forbush decreases in galactic cosmic rays caused by large-scale disturbances of the interplanetary medium for more than half a century has been created based on this series. The capabilities of the database make it possible to perform a correlation analysis of various parameters of the space environment (characteristics of the Sun, solar wind, and interplanetary magnetic field) with the parameters of cosmic rays and to study their interrelationships in the solar–terrestrial space. The features of reception coefficients for different stations are considered, which allows the transition from variations according to ground measurements to variations of primary cosmic rays. The advantages and disadvantages of this variant of the global survey method and the opportunities for its development and improvement are assessed. The developed method makes it possible to minimize the problems of the network of neutron monitors and to make significant use of its advantages.

Cosmic-Ray Anisotropy and the Local Interstellar Turbulence

We study the role of local interstellar turbulence in shaping the large-scale anisotropy in the arrival directions of TeV–PeV cosmic-rays (CRs) on the sky. Assuming pitch-angle diffusion of CRs in a magnetic flux tube containing the Earth, we compute the CR anisotropy for Goldreich-Sridhar turbulence, and for isotropic fast modes. The narrow deficits in the 400 TeV and 2 PeV data sets of IceTop can be fitted for some parameters of the turbulence. The data also rule out a part of the parameter space. The shape of the CR anisotropy may be used as a local probe of the still poorly known properties of the interstellar turbulence and of CR transport.

Unusual Cosmic Ray Variations During the Forbush Decreases of June 2015

Although the current Solar Cycle 24 is characterized by low solar activity, an intense geomagnetic storm (G4) was recorded in June 2015. It was a complex phenomenon that began on 22 June 2015 as the result of intense solar activity, accompanied by several flares and coronal mass ejections that interacted with the Earth’s magnetic field. A Forbush decrease was also recorded at the neutron monitors of the worldwide network, with an amplitude of 8.4%, and in its recovery phase, a second Forbush decrease followed, with an amplitude of 4.0% for cosmic rays of 10 GV obtained with the global survey method. The Dst index reached a minimum value of −204 nT that was detected on 23 June 2015 at 05:00 – 06:00 UT, while the Kp index reached the value eight. For our analysis, we used hourly cosmic-ray intensity data recorded by polar, mid-, and high-latitude neutron monitor stations obtained from the High Resolution Neutron Monitor Database. The cosmic-ray anisotropy variation at the ecliptic plane was also estimated and was found to be highly complex. We study and discuss the unusual and complex cosmic-ray and geomagnetic response to these solar events.

The Global Survey Method Applied to Ground-level Cosmic Ray Measurements

The global survey method (GSM) technique unites simultaneous ground-level observations of cosmic rays in different locations and allows us to obtain the main characteristics of cosmic-ray variations outside of the atmosphere and magnetosphere of Earth. This technique has been developed and applied in numerous studies over many years by the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN). We here describe the IZMIRAN version of the GSM in detail. With this technique, the hourly data of the world-wide neutron-monitor network from July 1957 until December 2016 were processed, and further processing is enabled upon the receipt of new data. The result is a database of homogeneous and continuous hourly characteristics of the density variations (an isotropic part of the intensity) and the 3D vector of the cosmic-ray anisotropy. It includes all of the effects that could be identified in galactic cosmic-ray variations that were caused by large-scale disturbances of the interplanetary medium in more than 50 years. These results in turn became the basis for a database on Forbush effects and interplanetary disturbances. This database allows correlating various space-environment parameters (the characteristics of the Sun, the solar wind, et cetera) with cosmic-ray parameters and studying their interrelations. We also present features of the coupling coefficients for different neutron monitors that enable us to make a connection from ground-level measurements to primary cosmic-ray variations outside the atmosphere and the magnetosphere. We discuss the strengths and weaknesses of the current version of the GSM as well as further possible developments and improvements. The method developed allows us to minimize the problems of the neutron-monitor network, which are typical for experimental physics, and to considerably enhance its advantages.