Sectorial anisotropy of solar cosmic rays

A hitherto unobserved sectorial pattern of anisotropy that was limited to a narrow and stable region was displayed by the ground level event (GLE) of January 24, 1971. For the entire 11/2 h interval following onset before isotropy set in, the anisotropy was limited to a 10° cone centered about 60° from the spiral magnetic field line. It is also the first solar particle event for which it is possible, by analytical procedures based upon a theoretical propagation model, to distinguish between two rival candidates for the parent flare.This research was sponsored by the National Science Foundation and Air Force Cambridge Research Laboratories, Office of Aerospace Research, under Contract No. F19628-70-C-0190, but the report does not necessarily reflect endorsement by the sponsor.     

Influence of solar heliospheric parameters on cosmic rays anisotropy

In the present work the cosmic ray data of three different neutron monitoring stations, Deep River, Inuvik, and Tokyo, located at different geomagnetic cutoff rigidities and altitudes have been harmonically analyzed for the period 1980–95 for a comparative study of diurnal semi-diurnal and tri-diurnal anisotropies in cosmic ray intensity in connection with the change in interplanetary magnetic field Bz component and solar wind velocity on 60 quietest days. It is observed that the amplitudes of all the three harmonics increase during the period 1982–84 at all the stations during the high speed solar wind stream epoch and remain low during the declining phase of the stream. The amplitudes of the three harmonics have no obvious characteristics associated with the time variation of magnitude of the Bz component. The phases of all the three harmonics have no time variation characteristics associated with solar wind velocity and Bz. Published in Astrofizika, Vol. 49, No. 4, pp. 651–664 (August 2006).     

A search for small-scale anisotropy of PeV cosmic rays

Recent results of Milagro, Tibet, ARGO-YBJ, and IceCube experiments on the small-scale anisotropy of Galactic cosmic rays (CRs) with energies from units up to a few hundred TeV arise a question on a possible nature of the observed phenomenon, as well as on the anisotropy of CRs at higher energies. An analysis of a small-scale anisotropy of CRs with energies at around PeV registered with the EAS MSU array presented in the article, reveals a number of regions with an excessive flux. A typical size of the regions varies from 3° up to 12°. We study correlation of these regions with positions of potential astrophysical sources of CRs and discuss a possible origin of the observed anisotropy.     

On the latitude dependence of the semidiurnal anisotropy of cosmic rays

The expected diurnal waves with different harmonics in cosmic ray intensity arising from the semidiurnal anisotropy, due to the geometrical inclination of the Earth's axis, are calculated for different cosmic ray stations. The sensitivity of these waves to the exponent n of the latitude dependence function cosⁿ λ for the semidiurnal anisotropy is investigated. The amplitudes of the geometrical tridiurnal waves for high latitude stations show a great sensitivity to n and, therefore, it is concluded that its value can be determined precisely from the tridiurnal wave rather than from the semidiurnal waves observed at different latitudes. Available data from high latitude neutron monitors were used to determine n and it was found as 2±0.4, which is of higher accuracy than the previously determined values. The present results are consistent with either the density gradient or loss cone models of the semidiurnal anisotropy. Furthermore, they show that the geometrical tridiurnal waves have a very small amplitude and can be neglected in any analysis concerning tridiurnal variations in cosmic ray intensity.     

Reconstruction of the direction of the true anisotropy of galactic cosmic rays

A method of reconstructing the declination of galactic cosmic ray anisotropy is described, and its results are presented. The method is based on analysis of delay distributions in symmetrically arranged detectors of an air shower array, and it represents a modification of the crossed telescopes method. It is shown that the declination of the true anisotropy vector is close to 60° (i.e., this vector lies approximately within the galactic plane). Because of this, the true degree of anisotropy of galactic cosmic rays is severalfold higher than the first harmonic of intensity in the sidereal time (the quantity measured directly), and it equals about 0.2%.     

On the magnitude and direction of the anisotropy of galactic cosmic rays

The results of measuring the diurnal cosmic-ray intensity variations in the energy range 1–100 TeV are discussed. Whereas the phase of the first harmonic of the sidereal daily wave directly determines the phase (right ascension) of the cosmic-ray anisotropy vector, the amplitude and declination of the true anisotropy cannot be reconstructed directly from the amplitude of the first harmonic. However, they can be determined by invoking data on the zero harmonic. The results of some recent experiments purporting to measure the cosmic-ray anisotropy with a particularly high accuracy are shown to be interpreted erroneously.     

Understanding galactic cosmic rays

The case is made for most cosmic rays having come from galactic sources. ‘Structure’, i.e. a lack of smoothness in the energy spectrum, is apparent, strengthening the view that most cosmic rays come from discrete sources, supernova remnants being most likely.     

Relativistic heavy cosmic rays

During three balloon flights of a 1 m² sr ionizationchamber erenkov counter detector system, we have measured the atmospheric attenuation, flux, and charge composition of cosmic-ray nuclei with 16Z30 and rigidity greater than 4.5 GV.The attenuation mean-free-path in air of VH (20Z30) nuclei is found to be 19.7±1.6 g cm^–2, a value somewhat greater than the best previous measurement. The attenuation mean-free-path of iron is found to be 15.6±2.2 g cm^–2, consistent with predictions of geometric cross-section formulae.We measure an absolute flux of VH nuclei 10 to 20% higher than earlier experiments at similar geomagnetic cutoff and level of solar activity. The relative abundances of evencharged nuclei are found to be in good agreement with results of other recent high-resolution counter experiments.We calculate that our observed cosmic ray chemical composition implies relative abundances at the cosmic-ray source of Ca/Fe=0.12±0.04 and S/Fe=0.14±0.05. The results are consistent with all other elements of charge between 16 and 26 being absent at the source and being produced by cosmic-ray fragmentation in interstellar hydrogen. The results show the ratios A/Fe and S/Fe to be significantly lower in the cosmic-ray source than in the solar system.     

On metagalactic cosmic rays

The simple argument is presented to show that the average energy density of cosmic rays in the Metagalaxy must be much smaller than in the Galaxy. This conclusion could, in principle, be not valid in the Lemaître cosmological model. The gamma-ray astronomical data now available testify, however, against the possibility of the cosmic-ray storage during the stop phase of the Lemaître model. The measurements of the diffuse background gamma-ray intensity with energy exceeding 10 MeV could definitely solve this problem.     

Secular changes in the upper cut-off rigidity of the solar diurnal anisotropy of cosmic rays

We have calculated the upper cut-off rigidities applicable to the solar diurnal anisotropy of cosmic rays, for the period 1965–1979. Our results are consistent with those reported by others. We note that the mean values of the upper cut-off rigidities during 1975–1977 are less than 50 GV. This explains why no diurnal variation is observed by the muon detectors at Socorro (NM) at a depth of 80 m of water equivalent (MWE) for this period. The threshold (R_o) and the median primary rigidities (R_m) of response applicable to the underground vertical telescope at Socorro are 45 and 300 GV, respectively.     

Anisotropy of high-energy cosmic rays

The anisotropy of high-energy (∼10 GeV and above) cosmic rays is investigated. A simplified model of the heliosphere constructed as a basis for the theory of their long-period variations is investigated for applicability to describing the cosmic-ray anisotropy. This model has been found to need a modification. The necessary changes of the model do not affect the conclusions on the 22-year variations reached on its basis but make it possible to construct a theory of cosmic-ray anisotropy. The theoretical results on the anisotropy are compared with its long-term observations in a wide energy range performed in Yakutsk.     

Sector structure of the interplanetary magnetic field and anisotropy of 50–1000 GV cosmic rays

It is demonstrated that, at high rigidities (50 GV and beyond), all the main features of cosmic-ray anisotropy of solar origin can be explained in terms of regular particle motion —without diffusion being involved — in the large-scale interplanetary magnetic field (IMF). A simple model of the IMF is adopted with a corotating warped neutral sheet separating the regions of alternative polarities; the warped shape is indispensable for obtaining any form of anisotropy. Energy losses occurring along various computed trajectories are calculated to give the sidereal, solar and antisidereal intensity waves. The reliability of the variations obtained are checked by changing the parameters of the IMF model. Both the sense and amplitude of the polarity-dependent sidereal vector are compatible with those established experimentally. Also reproduced are the predictions of corotation in addition to the 3-hour phase of the semi-diurnal wave. The corotation is found to be near perfect at 50 GV, while it reduces at 100 GV. The model presented accounts for the change of solar daily vector that was observed in 1969.     

Gamma rays from cosmic radioactivities

Abstract— Gamma rays from radioactive byproducts of cosmic nucleosynthesis are direct messengers from nuclear processes taking place in various cosmic sites, and can be measured with telescopes operated in space. Due to low detector sensitivity, up until now, only a handful of sources have been detected in that electromagnetic window. Cobalt lines from SN1987A and ⁴⁴Ti lines from the Cassiopeia A (Cas A) supernova remnant offer unique constraints on the properties of the innermost regions of core collapse supernovae. Diffuse gamma‐ray lines from the decay of radioactive ²⁶Al and the annihilation of positrons are bright enough for mapping the Milky Way in the MeV regime, and are both measured by recent spaceborne spectrometers with unprecedented precision. This constrains the sources of Al production and the state of interstellar gas in the vicinity of these sites: the total mass of ²⁶Al produced by stellar sources throughout the Galaxy is estimated to be ～3 M_⊙ per Myr, and the interstellar medium near those sources appears to be characterized by velocities of ～100 km s^?1. Positron annihilation must occur in a modestly ionized, warm phase of the interstellar medium, but at present the major positron production site(s) remain unknown. The spatial distribution of the annihilation gamma‐ray emission constrains positron production sites and positron propagation in the Galaxy. ⁶⁰Fe radioactivity has been clearly detected recently; the flux ratio relative to ²⁶Al of about 15% is on the lower side of predictions from massive star and supernova nucleosynthesis models. Those views at nuclear and astrophysical processes in and around cosmic sources by space‐based gamma‐ray telescopes offer invaluable information on cosmic nucleosynthesis.     

Cosmic rays and cosmic strings

It has been suggested that the highest-energy cosmic rays might be protons resulting from collapsing cosmic strings in the Universe. We point out that this mechanism, although attractive, has important shortcomings, notably the fact that gamma rays produced along with the protons and those produced by the protons in their interactions with the cosmic background radiation generate cascades in the Universe and result in unacceptably high fluxes of cosmic gamma rays in the region of hundreds of MeV.     

Shock acceleration of solar cosmic rays

The solar cosmic ray (SCR) acceleration by the shocks driven by coronal mass ejections is studied by taking into account the generation of Alfvén waves by accelerated particles. Detailed numerical calculations of the SCR spectra produced during the shock propagation through the solar corona have been performed within a quasi-linear approach with a realistic set of coronal parameters. The resultant SCR energy spectrum is shown to include a power-law part N ∝ ?^-γ with an index γ = 1.7–3.5 that ends with an exponential tail. The maximum SCR energy lies within the range ? _max = 0.01–10 GeV, depending on the shock velocity V _S = 750–2500 km s^?1. The decrease of the shock Alfvénic Mach number due to the increase Alfvén velocity with heliocentric distance r leads to the end of the efficient SCR acceleration when the shock size reaches R _S ≈ 4R _⊙. In this case, the diffusive SCR propagation begins to exceed the shock velocity; as a result, SCRs escape intensively from the shock vicinity. The self-consistent generation of Alfvén waves by accelerated particles is accompanied by a steepening of the particle spectrum and an increase of their maximum energy. Comparison of the calculated SCR fluxes expected near the Earth’s orbit with the available experimental data shows that the theory explains the main observed features.     

On the acceleration of cosmic rays

The intensive acceleration of energetic charged particles in perpendicular shock waves which has been known to take place in the interplanetary medium has been utilized in this work in order to account for the energization of cosmic rays. It is proposed that cosmic rays can be accelerated up to 10¹⁴–10¹⁵ eV in successive perpendicular shock waves which appear inside supernova shells in our Galaxy.     

Homogeneous metagalactic origin of cosmic rays

Starting with the hypothesis that cosmic rays are evenly distributed in the metagalaxy, it is shown that the flux of the electron-positron component, which is produced through --e decays, following the nuclear collisions of the cosmic ray beam with the intergalactic medium, takes <-4×10¹⁶ sec to reach steady state. The corresponding value of the flux of thepositron component and its implications regarding the homogeneous model of the metagalactic origin of cosmic rays are discussed.On leave from Tata Institute of Fundamental Research, Bombay, India.     

Shock acceleration of solar cosmic rays

We investigated the acceleration of solar cosmic rays (SCRs) by the shock waves produced by coronal mass ejections. We performed detailed numerical calculations of the SCR spectra produced during the shock propagation in the solar corona in terms of a model based on the diffusive transport equation using a realistic set of physical parameters for the corona. The resulting SCR energy spectrum N(ε) ∝ ε^?γ exp [? (ε/ε_max)^α] is shown to include a power-law portion with an index γ?2 that ends with an exponential tail with α ? 2.5 ? β, where β is the spectral index of the background Alfvén turbulence. The maximum SCR energy lies within the range ε_max = 1–300 MeV, depending on the shock velocity. Because of the steep spectrum of the SCRs, their backreaction on the shock structure is negligible. The decrease in the Alfvén Mach number of the shock due to the increase in the Alfvén velocity with heliocentric distance r causes the efficient SCR acceleration to terminate when the shock reaches a distance of r = 2–3R_⊙. Since the diffusive SCR propagation in this case is faster than the shock expansion, SCR particles intensively escape from the shock vicinity. A comparison of the calculated SCR fluxes expected near the Earth’s orbit with available experimental data indicates that the theory satisfactorily explains all of the main observed features.     

Anisotropic diffusion of solar cosmic rays

A simple model is described for solar cosmic ray events which appears to be in reasonable accord with observations. The model is based partly on some earlier models, together with the assumption that the diffusion of particles is strongly anisotropic due to the presence of the interplanetary magnetic field. Some remarks concerning the limitations of the diffusion equation are included, and it is pointed out that the propagation of solar cosmic rays might be best described in terms of an analogy to electrical transmission lines rather than to the conduction of heat as is usually done.