Cosmic Ray Anisotropy Research Articles

Forbush decreases associated with coronal mass ejections from active and non-active regions: statistical comparison

ABSTRACT In this paper, Forbush decreases (FDs) from 1997 to 2020 associated with coronal mass ejections from active and non-active regions are compared between themselves and to FDs caused by high-speed streams from coronal holes. The two types of sporadic FDs are also compared when corresponding solar wind (SW) disturbances contain, or do not contain, magnetic clouds (MCs) near Earth. Cosmic ray density and anisotropy variations, SW speed, interplanetary magnetic field (IMF) strength, and geomagnetic indices have been examined using statistical methods. The results reveal that these parameters are larger for FDs associated with active region (AR) ejections and have highly skewed distributions for both types of sporadic events. In the same ranges of SW parameters, FD magnitude is larger for flare-associated events; more efficient modulation occurs in FDs associated with AR ejections. Differences between FDs associated with AR and non-AR ejections are more pronounced when an MC is registered. For IMF strength and geomagnetic indices, differences between the distributions depend more upon MC presence or absence than on the type of solar source. Correlation of IMF strength and SW speed differs slightly between FDs caused by AR and non-AR ejections regardless of the presence or absence of an MC, akin to the partial correlation between FD magnitude and IMF strength. Difference between the speeds of disturbed and background SW is larger for FDs associated with AR ejections especially when an MC is registered; the interaction region of different-speed SW streams occurs more frequently in interplanetary disturbances induced by AR ejections.

Pitch-angle anisotropy and differential rigidity spectra of cosmic rays during GLE on May 2 and 6, 1998

Using data from the worldwide network of neutron monitors (39 stations) and the method of global spectrographic survey, we have studied pitch-angle anisotropy and differential rigidity spectra of cosmic rays during the ground level enhancements on May 2 and 6, 1998. We obtained differential rigidity spectra of solar cosmic rays in these events and determined the maximum rigidities to which protons accelerated. The maximum rigidities of accelerated protons during the ground level enhancements on May 2 was ~2.4; on May 6, ~1.8 GV. The revealed bidirectional pitch-angle anisotropy indicates that Earth was in the IMF loop structure during these events.

Питч-угловая анизотропия и дифференциальные жесткостные спектры космических лучей во время GLE 2 и 6 мая 1998 г.

Using data from the worldwide network of neutron monitors (39 stations) and the method of global spectrographic survey, we have studied pitch-angle anisotropy and differential rigidity spectra of cosmic rays during the ground level enhancements on May 2 and 6, 1998. We obtained differential rigidity spectra of solar cosmic rays in these events and determined the maximum rigidities to which protons accelerated. The maximum rigidities of accelerated protons during the ground level enhancements on May 2 was ~2.4; on May 6, ~1.8 GV. The revealed bidirectional pitch-angle anisotropy indicates that Earth was in the IMF loop structure during these events.

Interpretation of the Spectra and Anisotropy of Galactic Cosmic Rays

Recent measurements of the spectra and anisotropy of cosmic rays (CRs) show a fine structure that reflects the spectral hardenings of CRs nuclei at the rigidity R ∼ 200 GV followed by softenings at R ∼ 10 TV, and reveal complicated energy dependence of the amplitude and phase of anisotropy from 100 GeV to PeV. Numerous studies have shown that the existence of nearby CR sources and a local interstellar magnetic field (LIMF) near the solar system are crucial for such CR spectral and anisotropic patterns. In this work, we analyze the CR spectra of different CR components and the anisotropy considering the nearby Geminga supernova remnants (SNRs) source. In the calculation process, we also introduce the anisotropic diffusion of CRs induced by the LIMF based on the spatial-dependent propagation (SDP) model. As a result, our model can simultaneously account for the CR spectra and the anisotropy from 100 GeV to PeV. Future high-precision measurements of the CR anisotropy, for example, by the LHAASO experiment, would be of the essence in the assessment of our proposed model.

Testing the Empirical Relationship between Forbush Decreases and Cosmic Ray Diurnal Anisotropy

The abrupt aperiodic modulation of cosmic ray (CR) flux intensity, often referred to as Forbush decrease (FD), plays a significant role in our understanding of the Sun–Earth electrodynamics. Accurate and precise determinations of FD magnitude and timing are among the intractable problems in FD-based analysis. FD identification is complicated by CR diurnal anisotropy. CR anisotropy can increase or reduce the number and amplitude of FDs. It is therefore important to remove its contributions from CR raw data before FD identification. Recently, an attempt was made, using a combination of the Fourier transform technique and FD-location machine, to address this. Thus, two FD catalogs and amplitude diurnal variation (ADV) were calculated from filtered (FD1 and ADV) and raw (FD2) CR data. In the current work, we test the empirical relationship between FD1, FD2, ADV and solar-geophysical characteristics. Our analysis shows that two types of magnetic fields - interplanetary and geomagnetic (Dst) - govern the evolution of CR flux intensity reductions.

A proposal to extend the spectrographic global survey method

We present a modified spectrographic global survey method to split the cosmic ray variations into components of the interplanetary, geomagnetospheric, and atmospheric origin from ground-based observations of the cosmic ray intensity. We show a possibility to use all the available suite of ground-based instrumentation recording cosmic rays (global network of the neutron monitors located at different locations and altitudes, ground- and underground-based muon telescopes, etc.) for such studies without involving the data from aerologic atmospheric sounding. As a demonstration of the method functionality, we provide the calculation results for the variations in the isotropic flux, pitch angle anisotropy of primary cosmic rays in the interplanetary space, changes in the planetary system of geomagnetic cutoff rigidities for every observational hour, as well as the atmosphere temperature above the point of observation of cosmic ray charged components for selected time intervals.

Overview of the HERD space mission

The High Energy cosmic Radiation Detector (HERD) is a prominent space-borne instrument to be installed on-board the Chinese Space Station (CSS) around 2027, resulting from a collaboration among Chinese and European institutions. Primary scientific goals of HERD include: precise measurements of the cosmic ray (CR) energy spectra and mass composition at energies up to few PeV, electron/positron spectra up to tens of TeV, CR anisotropy, gamma ray astronomy and transient studies, along with indirect searches for Dark Matter candidates. The detector is configured to accept incident particles from both its top and four lateral sides. Owing to its pioneering design, more than one order of magnitude increase in geometric acceptance is foreseen, with respect to previous and ongoing experiments. HERD is conceived around a deep (∼55 X 0, 3 λ I ) 3D cubic calorimeter (CALO), forming an octagonal prism. Fiber Trackers (FiTs) are instrumented on all active sides, with a Plastic Scintillator Detector (PSD) covering the calorimeter and tracker. Ultimately, a Silicon Charge Detector (SCD) envelops the above-stated sub-detectors, while a Transition Radiation Detector (TRD) is instrumented on one of its lateral faces, for energy calibration in the TeV scale. This work illustrates HERD’s latest advancements and scientific objectives along with an overview of upcoming activities.

Peculiarities of sporadic variations in density and anisotropy of galactic cosmic rays in solar cycle 24

In this work, we have processed data from the global network of neutron monitors and muon telescopes by the global survey method to study variations in the density and anisotropy of galactic cosmic rays during Forbush decreases observed in solar cycle 24. The simultaneous use of two different type detectors made it possible to examine the temporal dynamics of the angular distribution of cosmic rays in two different energy intervals. Besides, we have used measurements of the Yakutsk cosmic ray spectrograph after A.I. Kuzmin to assess the energy spectrum index during large disturbances of the interplanetary medium in this cycle. Analysis of the results obtained confirms our early statements that solar activity cycle 24 features an increased level of turbulence in the interplanetary magnetic field.

Особенности спорадических вариаций плотности и анизотропии галактических космических лучей в 24-м цикле солнечной активности

In this work, we have processed data from the global network of neutron monitors and muon telescopes by the global survey method to study variations in the density and anisotropy of galactic cosmic rays during Forbush decreases observed in solar cycle 24. The simultaneous use of two different type detectors made it possible to examine the temporal dynamics of the angular distribution of cosmic rays in two different energy intervals. Besides, we have used measurements of the Yakutsk cosmic ray spectrograph after A.I. Kuzmin to assess the energy spectrum index during large disturbances of the interplanetary medium in this cycle. Analysis of the results obtained confirms our early statements that solar activity cycle 24 features an increased level of turbulence in the interplanetary magnetic field.

Anisotropies of ultrahigh-energy cosmic rays in a scenario with nearby sources

The images of ultrahigh-energy cosmic ray sources get distorted, in an energy dependent way, by the effects of Galactic and extragalactic magnetic fields. These deflections can also affect the observed cosmic ray spectrum, especially when the sources are transient. We study scenarios in which one or a few nearby extragalactic sources, such as Cen A or M81/M82, provide the dominant contribution to the cosmic ray flux above the ankle of the spectrum. We discuss the effects of the angular dispersion induced by the turbulent extragalactic magnetic fields, and the coherent deflections caused by the regular Galactic magnetic field, with the associated multiple imaging of the sources. We consider the possible contribution from those sources to the dipolar distribution discovered by the Pierre Auger Observatory above 8 EeV, as well as to the hot spots hinted in the observations by the Pierre Auger and Telescope Array observatories at higher energies, taking into account the mixed nature of the cosmic ray composition.

CrSbSe3 : A pseudo one-dimensional ferromagnetic semiconductor

Low-dimensional magnetic materials have attracted much attention due to their novel properties and high potential for spintronic applications. In this work, we study the electronic structure and magnetic properties of the pseudo one-dimensional compound CrSbSe$_3$, using density functional calculations, superexchange model analyses, and Monte Carlo simulations. We find that CrSbSe$_3$ is a ferromagnetic (FM) semiconductor with a band gap of about 0.65 eV. The FM couplings within each zig-zag spin chain are due to the Cr-Se-Cr superexchange with the near-90$^\circ$ bonds, and the inter-chain FM couplings are one order of magnitude weaker. By inclusion of the spin-orbit coupling (SOC) effects, our calculations reproduce the experimental observation of the easy magnetization $a$ axis and the hard $b$ axis (the spin-chain direction), with the calculated moderate magnetic anisotropy of 0.19 meV/Cr. Moreover, we identify the nearly equal contributions from the single ion anisotropy of Cr, and from the exchange anisotropy due to the strong SOC of the heavy elements Sb and Se and their couplings with Cr. Using the parameters of the magnetic coupling and anisotropy from the above calculations, our Monte Carlo simulations yield the Curie temperature $T_{\rm{C}}$ of 108 K.

Constraints on the spatially dependent cosmic-ray propagation model from Bayesian analysis

The energy spectra of primary and secondary cosmic rays (CRs) generally harden at several hundreds of GeV, a finding which can be naturally interpreted by means of propagation effects. We adopt a spatially dependent CR propagation model to fit the spectral hardening, where a slow-diffusion disk (SDD) is assumed near the galactic plane. We aim to constrain the propagation parameters with a Bayesian parameter estimation based on a Markov chain Monte Carlo sampling algorithm. The latest precise measurements of carbon spectrum and B/C ratio are adopted in the Bayesian analysis. The $^{10}\mathrm{Be}/^{9}\mathrm{Be}$ and Be/B ratios are also included to break parameter degeneracies. The fitting result shows that all the parameters are well constrained. Especially, the thickness of the SDD is limited to 0.4--0.5 kpc above and below the galactic plane, which could be the best constraint for the slow-diffusion region among similar works. The $\overline{p}/p$ ratio and the amplitude of CR anisotropy predicted by the SDD model are consistent with the observations, while the predicted high-energy electron and positron fluxes are slightly and significantly lower than the observations, respectively, indicating the necessity of extra sources.

A consistent reduced-speed-of-light formulation of cosmic ray transport valid in weak- and strong-scattering regimes

ABSTRACT We derive a consistent set of moment equations for cosmic ray (CR)-magnetohydrodynamics, assuming a gyrotropic distribution function (DF). Unlike previous efforts, we derive a closure, akin to the M1 closure in radiation hydrodynamics (RHD), that is valid in both the nearly isotropic DF and/or strong-scattering regimes, and the arbitrarily anisotropic DF or free-streaming regimes, as well as allowing for anisotropic scattering and transport/magnetic field structure. We present the appropriate two-moment closure and equations for various choices of evolved variables, including the CR phase space DF f, number density n, total energy e, kinetic energy ϵ, and their fluxes or higher moments, and the appropriate coupling terms to the gas. We show that this naturally includes and generalizes a variety of terms including convection/fluid motion, anisotropic CR pressure, streaming, diffusion, gyro-resonant/streaming losses, and re-acceleration. We discuss how this extends previous treatments of CR transport including diffusion and moment methods and popular forms of the Fokker–Planck equation, as well as how this differs from the analogous M1-RHD equations. We also present two different methods for incorporating a reduced speed of light (RSOL) to reduce time-step limitations: In both, we carefully address where the RSOL (versus true c) must appear for the correct behaviour to be recovered in all interesting limits, and show how current implementations of CRs with an RSOL neglect some additional terms.

Cosmic rays and non-thermal emission in simulated galaxies – III. Probing cosmic-ray calorimetry with radio spectra and the FIR–radio correlation

ABSTRACT An extinction-free estimator of the star formation rate (SFR) of galaxies is critical for understanding the high-redshift universe. To this end, the nearly linear, tight correlation of far-infrared (FIR), and radio luminosity of star-forming galaxies is widely used. While the FIR is linked to massive star formation, which also generates shock-accelerated cosmic-ray (CR) electrons and radio synchrotron emission, a detailed understanding of the underlying physics is still lacking. Hence, we perform three-dimensional magnetohydrodynamical (MHD) simulations of isolated galaxies over a broad range of halo masses and SFRs using the moving-mesh code arepo, and evolve the CR proton energy density self-consistently. In post-processing, we calculate the steady-state spectra of primary, shock-accelerated and secondary CR electrons, which result from hadronic CR proton interactions with the interstellar medium. The resulting total radio luminosities correlate with the FIR luminosities as observed and are dominated by primary CR electrons if we account for anisotropic CR diffusion. The increasing contribution of secondary emission up to 30 per cent in starbursts is compensated by the larger bremsstrahlung and Coulomb losses. CR electrons are in the calorimetric limit and lose most of their energy through inverse Compton interactions with star light and cosmic microwave background (CMB) photons while less energy is converted into synchrotron emission. This implies steep steady-state synchrotron spectra in starbursts. Interestingly, we find that thermal free–free emission flattens the total radio spectra at high radio frequencies and reconciles calorimetric theory with observations while free–free absorption explains the observed low-frequency flattening towards the central regions of starbursts.

Detecting ultra-high-energy cosmic ray anisotropies through harmonic cross-correlations

We propose an observable for ultra-high-energy cosmic ray (UHECR) physics: the harmonic-space cross-correlation power spectrum between the arrival directions of UHECRs and the large-scale cosmic structure mapped by galaxies. This cross-correlation has not yet been considered in the literature, and it permits a direct theoretical modelling of the main astrophysical components. We describe the expected form of the cross-correlation and show how, if the distribution of UHECR sources traces the large-scale cosmic structure, it could be easier to detect with current data than the UHECR auto-correlation. Moreover, the cross-correlation is more sensitive to UHECR anisotropies on smaller angular scales, it is more robust to systematic uncertainties, and it could be used to determine the redshift distribution of UHECR sources, making it a valuable tool for determining their origins and properties.

Ions Measured by Voyager 1 Outside the Heliopause to ~28 au and Implications Thereof

Abstract We report 40–139 keV in situ ion measurements from the Voyager 1/Low Energy Charged Particle (LECP) instrument, over the time period from 2000 to the first 3 months of 2020, toward assessing energetic ion flow properties inside the heliosheath (HS) and upstream of the heliopause (HP). We identify an average radial outflow of ions at ∼46, 67, and 109 keV of ∼2.33 × 10−3, ∼1.37 × 10−3, and ∼7.27 × 10−4 ( cm 2 sr s keV ) –1, respectively, from the beginning of 2013 up to 2020, i.e., over a spatial range of ∼28 au past the HP. The raw 40–139 keV rates, uncorrected for background, show distinct intensity dropouts for particles with 90° pitch angles, in concert with the >211 MeV Galactic cosmic-ray anisotropy events, reported after Voyager 1 exited the HS in 2012 August. The intensity spectrum is consistent with a power-law form in energy with a spectral index of ∼−1.4, similar to energy spectra measured by both Voyager 1 and 2/LECP inside the HS, providing strong indication that these particles correspond to an ion population leaking from the HS into interstellar space, most likely due to the flux tube interchange instability at the boundary. Future modeling efforts should incorporate this ion population, which provides the framework for establishing a communication between the HS and the very local interstellar medium.

Amplitude of the Usual Cosmic Ray Diurnal and Enhanced Anisotropies: Implications for the Observed Magnitude, Timing, and Ranking of Forbush Decreases

Abstract Cosmic ray (CR) diurnal anisotropy and Forbush decreases (FDs), as well as the relationship between them, have received considerable study. Several astrophysicists have focused on the speculated impact of FDs on the amplitude of the diurnal CR variations on different timescales. In an attempt to disentangle the contribution of FDs, days of Forbush events are traditionally excluded while calculating the diurnal amplitude in individual neutron monitor (NM) data. But the implications of CR diurnal anisotropy on the magnitude of FDs are rarely investigated in detail. Recently, an effort was made, using a combination of Fast Fourier transform and FD-location algorithms, to account for the contribution of CR diurnal anisotropy on the number, magnitude, and timing of FDs. With some technical advancements, the efficiency of the software is tested in the current work using CR data measured by the oldest CR observatory (Climax) from 1953 to 2006. We find strong and statistically significant correlations between FD magnitude calculated from raw and Fourier transformed CR data and the amplitude of diurnal anisotropy. The relationship is stronger in the case of the Fourier transformed signal, lending credence to the idea that CR anisotropy is a part of Forbush events. In order to validate the observed relationship between the amplitude of CR diurnal variations, magnitude, and timing of FDs, large volumes of data from the Moscow, McMurdo, and Potchefstroom NMs were also analyzed. The significant CR intensity decreases (FD magnitude >5%) at the four stations were ranked according to their magnitudes.

The Role of Jet–Cocoon Mixing, Magnetization, and Shock Breakout in Neutrino and Cosmic-Ray Emission from Short Gamma-Ray Bursts

Abstract We perform general relativistic magnetohydrodynamic and relativistic magnetohydrodynamic simulations of weakly and highly magnetized gamma-ray burst (GRB) jets propagating in binary neutron star (BNS) merger ejecta. Using the simulations, we first find that mixing between the jet and cocoon, which is present in all types of jets, inhibits the formation of subphotospheric collisionless shocks. However, we show that a mild magnetization may lead to the formation of collisionless subshocks, which allow efficient proton acceleration. We consider shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and external shocks to provide the first estimate for neutrino and cosmic-ray (CR) signals from self-consistent simulations of GRBs in BNS mergers. We find that short GRBs do not produce detectable neutrino signals with current-day facilities. Shock breakout yields ∼10 PeV neutrinos at viewing angles ∼20°, independent of the jet magnetization. However, a neutrino signal from shock breakout is well below the detection limits of current detectors. Such a signal would allow a coincident neutrino–γ-ray detection, providing a testable prediction for shock breakout as a neutrino production site. Using the numerical modeling that fits GW 170817 afterglow emission, we find that blast waves in BNS mergers can account for 5%–10% of the Galactic CR luminosity in the PeV–EeV energy range. Based on these estimates, the observed level of CR anisotropy places a constraint on the distance of the latest Galactic BNS merger to ≲3 kpc.

Ground level enhancements of cosmic rays on October–November 2003

We study variations in the rigidity spectrum and anisotropy of cosmic rays (CRs) by using the data from satellite and ground-based observations within the worldwide network of cosmic-ray detectors through the global spectrographic survey. The studied period covers 28 and 29 October and 2 November 2003 (GLE65, GLE66, and GLE67). We determined CR differential rigidity spectra at individual hours of the investigated events. The maximal rigidity of accelerated protons in these GLEs is shown to equal ~ 4–14 GV.

Cosmic rays and non-thermal emission in simulated galaxies – II. γ-ray maps, spectra, and the far-infrared–γ-ray relation

ABSTRACT The γ-ray emission of star-forming (SF) galaxies is attributed to hadronic interactions of cosmic ray (CR) protons with the interstellar gas and contributions from CR electrons via bremsstrahlung and inverse Compton (IC) scattering. The relative importance of these processes in different galaxy types is still unclear. We model these processes in three-dimensional magnetohydrodynamical (MHD) simulations of the formation of isolated galactic discs using the moving-mesh code arepo, including dynamically coupled CR protons and adopting different CR transport models. We calculate steady-state CR spectra and also account for the emergence of secondary electrons and positrons. This allows us to produce detailed γ-ray maps, luminosities, and spectra of our simulated galaxies at different evolutionary stages. Our simulations with anisotropic CR diffusion and a low CR injection efficiency at supernovae (SNe; $\zeta_\mathrm{SN}=0.05$) can successfully reproduce the observed far-infrared (FIR)–γ-ray relation. Starburst galaxies are close to the calorimetric limit, where CR protons lose most of their energy due to hadronic interactions and hence, their γ-ray emission is dominated by neutral pion decay. However, in low SF galaxies, the increasing diffusive losses soften the CR proton spectra due to energy-dependent diffusion, and likewise steepen the pionic γ-ray spectra. In turn, IC emission hardens the total spectra and can contribute up to ∼40 per cent of the total luminosity in low SF galaxies. Furthermore, in order to match the observed γ-ray spectra of starburst galaxies, we require a weaker energy dependence of the CR diffusion coefficient, $D\propto E^{0.3}$, in comparison to Milky Way-like galaxies.