Cosmic Ray Fluxes in Present and Past Times

The data on primary cosmic ray fluxes at the top of the atmosphere are given for the period since 1937 till the present time. These data have been obtained from the regular cosmic ray flux measurements in the stratosphere and on the ground level. They have been used to find the relationship of cosmic ray fluxes with solar activity (sunspot number). On the basis of the deduced relationship the cosmic ray fluxes in the past have been recovered, as the sunspot number is known since 1500. The link between the smoothed data on Be-10 atom concentrations and cosmic ray fluxes is established which gives a possibility to calculate cosmic ray fluxes in the far past.     

Cosmic Ray Nonlinear Processes in Space Plasma: Applications to the Dynamic Heliosphere

Influence of cosmic ray pressure and kinetic stream instability on space plasma dynamics and magnetic structure are considered. It is shown that in the outer Heliosphere are important dynamics effects of galactic cosmic ray pressure on solar wind and interplanetary shock wave propagation as well as on the formation of terminal shock wave of the Heliosphere and subsonic region between Heliosphere and interstellar medium. Kinetic stream instability effects are important on distances more than 40–60 AU from the Sun: formation of great anisotropy of galactic cosmic rays in about spiral interplanetary magnetic field leads to the Alfven turbulence generation by non isotropic cosmic ray fluxes. Generated Alfven turbulence influences on cosmic ray propagation, increases the cosmic ray modulation, decreases the cosmic ray anisotropy and increases the cosmic ray pressure gradient in the outer Heliosphere (the later is also important for terminal shock wave formation). This revised version was published online in July 2006 with corrections to the Cover Date.     

Pionic photons and neutrinos from cosmic ray accelerators

Identifying the accelerators that produce the Galactic and extragalactic cosmic rays has been a priority mission of several generations of high energy gamma ray and neutrino telescopes; success has been elusive so far. Detecting the gamma-ray and neutrino fluxes associated with cosmic rays reaches a new watershed with the completion of IceCube, the first neutrino detector with sensitivity to the anticipated fluxes, and the construction of CTA, a ground-based gamma ray detector that will map and study candidate sources with unprecedented precision. In this paper, we revisit the prospects for revealing the sources of the cosmic rays by a multiwavelength approach; after reviewing the methods, we discuss supernova remnants, gamma ray bursts, active galaxies and GZK neutrinos in some detail.     

高海拔宇宙线观测站LHAASO概况

宇宙线发现百年以来,宇宙线起源仍然是一个谜.研究宇宙线起源主要在甚高能(VHE)伽马射线天文学和宇宙线物理学两个领域交叉展开.新一代高海拔宇宙线观测站(LHAASO)拥有高海拔、全天候和大规模优势,利用多种探测手段对宇宙线开展联合观测,大幅提升对伽马射线和宇宙线的鉴别能力.LHAASO将开展全天区伽马源扫描搜索以大量发现新伽马源,将获得30TeV以上伽马射线探测的最高灵敏度,将在宽达5个数量级的能量范围内精确测量宇宙线分成份能谱,为揭开宇宙线起源谜团给出重要判据.系统介绍了LHAASO的探测器结构、性能优势和科学目标.     

Introduction to Large High Altitude Air Shower Observatory (LHAASO)

Since the century discovery of cosmic ray, the origin of cosmic ray is always a mystery. The study on the origin of high-energy cosmic ray is in an interdiscipline between the very high-energy (VHE) gamma-ray astronomy and the cosmic ray physics. The Large High Altitude Air Shower Observatory (LHAASO) is a unique and new generation cosmic-ray station with the advantages of high altitude, all-weather, and large-scale. It takes the function of hybrid technology to detect cosmic rays and to upgrade greatly the resolving power between gamma rays and cosmic rays. The LHAASO is expected to make the full-sky survey to find new gamma-ray sources, to obtain the highest sensitivity of gamma-ray detection at the high energy band of > 30 TeV, and to make the very high precision measurement on the component energy spectra of cosmic rays in a broad energy range of 5 orders of magnitude, in order to provide the evidence for revealing the mystery of the origin of cosmic ray. This paper describes the detector structure, performance superiority and scientific motivation of the LHAASO.     

宇宙线直接探测进展概述

宇宙线从发现起至今已超过百年。在20世纪上半叶,大型粒子加速器技术成熟以前,对宇宙线的研究引领着基本粒子物理的发展,从宇宙线研究中取得的多项成果斩获诺贝尔奖。21世纪,宇宙线因其与极端高能的物理规律和暗物质等新物理现象联系密切而绽放出新的活力,宇宙线起源、加速、传播等相关的天文学及物理学问题也备受关注。简述了近年来在空间直接观测宇宙线实验方面取得的进展,以及其对理解宇宙线物理问题的推动。最后概述了中国在相关领域的研究历程和现状。     

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.     

Evidence for the existence of adiabatic energy loss in interplanetary space from observations of the decay of the February 25–March 2, 1969 series of solar cosmic ray events

The integral flux of low energy protons (> 10 MeV) observed by the University of New Hampshire cosmic ray detector aboard the Pioneer 9 spaceprobe has been compared with similar measurements of the near-Earth spacecraft Explorer 34 during the decay phase of the February 25–March 2, 1969 series of solar cosmic ray events. At this time the Pioneer 9 spaceprobe was 0.8 AU from the Sun and close to the Sun-Earth radial line. The ratio of integral fluxes as measured by the separated spacecraft can be calculated theoretically during the convective phase of the decay of these events and will depend on whether energy loss processes are operative. A comparison of the observed and theoretically calculated ratios suggests that the adiabatic energy loss process is operative.     

Influence of interplanetary interaction regions on geomagnetic disturbances and tropospheric circulation

Proposed solar wind-magnetosphere energy coupling functions are studied. An empirical formula proposed by Svalgaard (1977) is found to predict the geomagnetic activity quite well.

The influence of solar wind interaction regions on the tropospheric circulation, through a suggested cosmic ray mechanism, was investigated. The cosmic ray intensity at Earth clearly showed a decrease at the time of passage of an interaction region. It is suggested that the well-known dip in the Vorticity Area Index may be caused by an interaction-modulated decrease in cosmic ray intensity.     

Overview of Direct Measurements of Cosmic Rays

The history of cosmic ray studies can be traced back to the 1910s when Hess and other scientists first discovered them. Cosmic rays are very important laboratories of particle physics, and have led to many important discoveries of fundamental particles, such as the positrons, muons, pions, and a series of strange particles. Cosmic rays are nowadays the key probes of the extremely high-energy physics and dark matter particles. A brief review about the history and recent progresses of direct observations of cosmic rays is presented. In recent years, the new space-borne experiments such as PAMELA and AMS-02, as well as a few of balloon-borne experiments, have measured the energy spectra of cosmic rays very precisely, and revealed several new features/anomalies. Remarkable excesses of positron fraction in the total electron plus positron fluxes have been observed, which may be caused by the annihilation/decay of dark matter particles or by astrophysical pulsars. The cosmic ray antiprotons, which are expected to have the same secondary origin as that of positrons, do not show significant excesses compared with the background prediction. This result also constrains the modeling of the positron excesses. In addition, the spectral hardening above several hundred GeV of cosmic ray nuclei has been revealed. These results have important and interesting implications on our understandings of the origin, acceleration, and propagation of cosmic rays. In particular, China has launched the Dark Matter Particle Explorer (DAMPE) to indirectly search for the dark matter and explore the high-energy universe in the TeV window. Most recently, the DAMPE collaborators reported the new measurements of the cosmic ray electron plus positron fluxes up to about 5 TeV with a very high precision. The DAMPE data revealed clearly a deflection around 0.9 TeV in the electron energy spectrum. Possible fine structures of the electron plus positron spectra can be critically addressed with the accumulation of data in the coming years.     

Calibration of particle detectors for secondary cosmic rays using gamma-ray beams from thunderclouds

After observation of hundreds of Thunderstorm Ground Enhancements (TGEs) we measure energy spectra of particles originated in clouds and directed towards Earth. We use these “beams” for calibration of cosmic ray detectors located beneath the clouds at an altitude of 3200 m at Mount Aragats in Armenia. The calibrations of particle detectors with fluxes of TGE gamma rays are in good agreement with simulation results and allow estimation of the energy thresholds and efficiencies of numerous particle detectors used for studying galactic and solar cosmic rays.     

Spectrum and anisotropy of cosmic rays at TeV–PeV-energies and contribution of nearby sources

The role of nearby galactic sources, the supernova remnants, in formation of observed energy spectrum and large-scale anisotropy of high-energy cosmic rays is studied. The list of these sources is made up based on radio, X-ray and gamma-ray catalogues. The distant sources are treated statistically as ensemble of sources with random positions and ages. The source spectra are defined based on the modern theory of cosmic ray acceleration in supernova remnants while the propagation of cosmic rays in the interstellar medium is described in the frameworks of galactic diffusion model. Calculations of dipole component of anisotropy are made to reproduce the experimental procedure of “two-dimensional” anisotropy measurements. The energy dependence of particle escape time in the process of acceleration in supernova remnants and the arm structure of sources defining the significant features of anisotropy are also taken into account. The essential new trait of the model is a decreasing number of core collapse SNRs being able to accelerate cosmic rays up to the given energy, that leads to steeper total cosmic ray source spectrum in comparison with the individual source spectrum. We explained simultaneously the new cosmic ray data on the fine structure of all particle spectrum around the knee and the amplitude and direction of the dipole component of anisotropy in the wide energy range 1 TeV–1 EeV. Suggested assumptions do not look exotic, and they confirm the modern understanding of cosmic ray origin.     

Radio Cherenkov signals from the Moon: Neutrinos and cosmic rays

Neutrino production of radio Cherenkov signals in the Moon is the object of radio telescope observations. Depending on the energy range and detection parameters, the dominant contribution to the neutrino signal may come from interactions of the neutrino on the Moon facing the telescope, rather than neutrinos that have traversed a portion of the Moon. Using the approximate analytic expression of the effective lunar aperture from a recent paper by Gayley, Mutel and Jaeger, we evaluate the background from cosmic ray interactions in the lunar regolith. We also consider the modifications to the effective lunar aperture from generic non-standard model neutrino interactions. A background to neutrino signals are radio Cherenkov signals from cosmic ray interactions. For cosmogenic neutrino fluxes, neutrino signals will be difficult to observe because of low neutrino flux at the high energy end and large cosmic ray background in the lower energy range considered here. We show that lunar radio detection of neutrino interactions is best suited to constrain or measure neutrinos from astrophysical sources and probe non-standard neutrino-nucleon interactions such as microscopic black hole production.     

Non-linear amplification of a magnetic field driven by cosmic ray streaming

One-, two- and three-dimensional numerical results of the non-linear interaction between cosmic rays and a magnetic field are presented. These show that cosmic ray streaming drives large-amplitude Alfvénic waves. The cosmic ray streaming energy is very efficiently transferred to the perturbed magnetic field of the Alfvén waves, and the non-linear time-scale of the growth of the waves is found to be very rapid, of the order of the gyro-period of the cosmic ray. Thus, a magnetic field of interstellar values, assumed in models of supernova remnant blast wave acceleration, would not be appropriate in the region of the shock. The increased magnetic field reduces the cosmic ray acceleration time and so increases the maximum cosmic ray energy, which may provide a simple and elegant resolution to the highest energy Galactic cosmic ray problem, where the cosmic rays themselves provide the fields necessary for their acceleration.     

Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays?

The origin of cosmic rays is one of the long-standing mysteries in physics and astrophysics. Simple arguments suggest that a scenario of supernova remnants (SNRs) in the Milky Way as the dominant sources for the cosmic ray population below the knee could work: a generic calculation indicates that these objects can provide the energy budget necessary to explain the observed flux of cosmic rays. However, this argument is based on the assumption that all sources behave in the same way, i.e. they all have the same energy budget, spectral behavior and maximum energy. In this paper, we investigate if a realistic population of SNRs is capable of producing the cosmic ray flux as it is observed below the knee. We use 21 SNRs that are well-studied from radio wavelengths up to gamma-ray energies and derive cosmic ray spectra under the assumption of hadronic emission. The cosmic ray spectra show a large variety in their energy budget, spectral behavior and maximum energy. These sources are assumed to be representative for the total class of SNRs, where we assume that about 100–200 cosmic ray emitting SNRs should be present today. Finally, we use these source spectra to simulate the cosmic ray transport from individual SNRs in the Galaxy with the GALPROP code for cosmic ray propagation. We find that the cosmic ray budget can be matched well for these sources. We conclude that gamma-ray emitting SNRs can be a representative sample of cosmic ray emitting sources. In the future, experiments like CTA and HAWC will help to distinguish hadronic from leptonic sources and to further constrain the maximum energy of the sources and contribute to producing a fully representative sample in order to further investigate the possibility of SNRs being the dominant sources of cosmic rays up to the knee.     

Cosmic ray antiprotons from nearby cosmic accelerators

The antiproton flux measured by PAMELA experiment might have originated from Galactic sources of cosmic rays. These antiprotons are expected to be produced in the interactions of cosmic ray protons and nuclei with cold protons. Gamma rays are also produced in similar interactions inside some of the cosmic accelerators. We consider a few nearby supernova remnants observed by Fermi LAT. Many of them are associated with molecular clouds. Gamma rays have been detected from these sources which most likely originate in decay of neutral pions produced in hadronic interactions. The observed gamma ray fluxes from these SNRs are used to find out their contributions to the observed diffuse cosmic ray antiproton flux near the earth.     

Propagation of galactic cosmic rays in the outer heliosphere

The propagation of galactic cosmic rays in heliospheric magnetic fields is studied. An approximate solution to the cosmic ray transport equation has been derived on the basis of a method that takes into account the small value of anisotropy of particle angular distribution. The spatial and energy distributions of the cosmic ray intensity and anisotropy have been investigated, and estimates of cosmic ray energy flux have been carried out.     

Analytic calculations of the spectra of ultra high energy cosmic ray nuclei. II. The general case of background radiation

We discuss the problem of ultra high energy nuclei propagation in extragalactic background radiations. The present paper is the continuation of the accompanying paper I where we have presented three new analytic methods to calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both primary and secondary, and secondary protons. The computation scheme in this paper is based on the analytic solution of coupled kinetic equations, which takes into account the continuous energy losses due to the expansion of the universe and pair-production, together with photo-disintegration of nuclei. This method includes in the most natural way the production of secondary nuclei in the process of photo-disintegration of the primary nuclei during their propagation through extragalactic background radiations. In paper I, in order to present the suggested analytical schemes of calculations, we have considered only the case of the cosmic microwave background radiation, in the present paper we generalize this computation to all relevant background radiations, including infra-red and visible/ultra-violet radiations, collectively referred to as extragalactic background light. The analytic solutions allow transparent physical interpretation of the obtained spectra. Extragalactic background light plays an important role at intermediate energies of ultra high energy cosmic ray nuclei. The most noticeable effect of the extragalactic background light is the low-energy tail in the spectrum of secondary nuclei.     

Cosmic flux of low energy gamma rays

Measurements have been made on the cosmic gamma rays of energy between 0.25 and 4.2 MeV from a balloon experiment made near the geomagnetic equator using a collimated 7.6 cm×7.6 cm NaI(T1) crystal assembly. The depth-intensity curves obtained were used to estimate the contribution due to the diffuse cosmic gamma rays in the above energy interval; an unfolding of the counting rates was then performed to obtain the energy spectrum. It is found that a power law fitted to the present data points has a spectral index of –1.8±0.2. A critical examination is then made of all the observational data between 1 keV and 100 MeV to deduce information on the spectral shape in this energy region. Upper limits on low energy gamma ray fluxes from Sco X-1 and the Galactic centre region are also reported.     

Non-Linear Amplification of a Magnetic Field Driven by Cosmic Ray Streaming

One dimensional numerical results of the non-linear interaction between cosmic rays and a magnetic field are presented. These show that cosmic ray streaming drives large amplitude Alfvénic waves. The cosmic ray streaming energy is very efficiently transfered to the perturbed magnetic field of the Alfvén waves. Thus a magnetic field of interstellar values, assumed in models of supernova remnant blast wave acceleration, would not be appropriate in the region of the shock. The increased magnetic field reduces the acceleration time and so increases the maximum cosmic ray energy, which may provide a simple and elegant resolution to the highest energy galactic cosmic ray problem were the cosmic rays themselves provide the fields necessary for their acceleration. This revised version was published online in July 2006 with corrections to the Cover Date.