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 Physics , 1998, DOI: 10.1088/0954-3899/24/5/002 Abstract: Experimental results from Haverah Park, Yakutsk, AGASA and Fly's Eye are reviewed. All these experiments work in the energy range above 0.1 EeV. The 'dip' structure around 3 EeV in the energy spectrum is well established by all the experiments, though the exact position differs slightly. Fly's Eye and Yakutsk results on the chemical composition indicate that the cosmic rays are getting lighter over the energy range from 0.1 EeV to 10 EeV, but the exact fraction is hadronic interaction model dependent, as indicated by the AGASA analysis. The arrival directions of cosmic rays are largely isotropic, but interesting features may be starting to emerge. Most of the experimental results can best be explained with the scenario that an extragalactic component gradually takes over a galactic population as energy increases and cosmic rays at the highest energies are dominated by particles coming from extragalactic space. However, identification of the extragalactic sources has not yet been successful because of limited statistics and the resolution of the data.
 Fazeleh Khajenabi Physics , 2011, DOI: 10.1007/s10509-011-0829-0 Abstract: We present a linear perturbation analysis of the magnetorotational instability in the presence of the cosmic rays. Dynamical effects of the cosmic rays are considered by a fluid description and the diffusion of cosmic rays is only along the magnetic field lines. We show an enhancement in the growth rate of the unstable mode because of the existence of cosmic rays. But as the diffusion of cosmic rays increases, we see that the growth rate decreases. Thus, cosmic rays have a destabilizing role in the magnetorotational instability of the accretion discs.
 Physics , 1998, DOI: 10.1016/S0370-1573(99)00101-5 Abstract: Cosmic ray particles with energies in excess of 10**(20) eV have been detected. The sources as well as the physical mechanism(s) responsible for endowing cosmic ray particles with such enormous energies are unknown. This report gives a review of the physics and astrophysics associated with the questions of origin and propagation of these Extremely High Energy (EHE) cosmic rays in the Universe. After a brief review of the observed cosmic rays in general and their possible sources and acceleration mechanisms, a detailed discussion is given of possible "top-down" (non-acceleration) scenarios of origin of EHE cosmic rays through decay of sufficiently massive particles originating from processes in the early Universe. The massive particles can come from collapse and/or annihilation of cosmic topological defects (such as monopoles, cosmic strings, etc.) associated with Grand Unified Theories or they could be some long-lived metastable supermassive relic particles that were created in the early Universe and are decaying in the current epoch. The highest energy end of the cosmic ray spectrum can thus be used as a probe of new fundamental physics beyond Standard Model. We discuss the role of existing and proposed cosmic ray, gamma-ray and neutrino experiments in this context. We also discuss how observations with next generation experiments of images and spectra of EHE cosmic ray sources can be used to obtain new information on Galactic and extragalactic magnetic fields and possibly their origin.
 Physics , 2001, DOI: 10.1063/1.1398166 Abstract: We explore the feasibility of using the Moon as a detector of extremely high energy (>10^19 eV) cosmic rays and neutrinos. The idea is to use the existing radiotelescopes on Earth to look for short pulses of Cherenkov radiation in the GHz range emitted by showers induced just below the surface of the Moon when cosmic rays or neutrinos strike it. We estimate the energy threshold of the technique and the effective aperture and volume of the Moon for this detection. We apply our calculation to obtain the expected event rates from the observed cosmic ray flux and several representative theoretical neutrino fluxes.
 Murat Boratav Physics , 1996, DOI: 10.1016/0920-5632(96)00300-3 Abstract: Over the last 30 years or so, a handful of events observed in ground-based cosmic ray detectors seem to have opened a new window in the field of high-energy astrophysics. These events have energies exceeding 5x10**19 eV (the region of the so-called Greisen-Zatsepin-Kuzmin spectral cutoff); they seem to come from no known astrophysical source; their chemical composition is mostly unknown; no conventional accelerating mechanism is considered as being able to explain their production and propagation to earth. Only a dedicated detector can bring in the high-quality and statistically significant data needed to solve this long-lasting puzzle: this is the aim of the Auger Observatory project around which a world-wide collaboration is being mobilized.
 Physics , 2000, DOI: 10.1016/S0217-751X(00)00090-2 Abstract: Over the last third of the century, a few tens of events, detected by ground-based cosmic ray detectors, have opened a new window in the field of high-energy astrophysics. These events have macroscopic energies, unobserved sources, an unknown chemical composition and a production and transport mechanism yet to be explained. With a flux as low as one particle per century per square kilometer, only dedicated detectors with huge apertures can bring in the high-quality and statistically significant data needed to answer those questions. In this article, we review the present status of the field both from an experimental and theoretical point of view. Special attention is given to the next generation of detectors devoted to the thorough exploration of the highest energy ranges
 Physics , 1999, DOI: 10.1016/S0927-6505(99)00119-X Abstract: The arrival directions of extremely high energy cosmic rays (EHECR) above $4\times10^{19}$ eV, observed by four surface array experiments in the northern hemisphere,are examined for coincidences from similar directions in the sky. The total number of cosmic rays is 92.A significant number of double coincidences (doublet) and triple coincidences (triplet) are observed on the supergalactic plane within the experimental angular resolution. The chance probability of such multiplets from a uniform distribution is less than 1 % if we consider a restricted region within $\pm 10^{\circ}$ of the supergalactic plane. Though there is still a possibility of chance coincidence, the present results on small angle clustering along the supergalactic plane may be important in interpreting EHECR enigma. An independent set of data is required to check our claims.
 Physics , 1999, Abstract: We developed numerical code for calculation of the extragalactic component of the spectra of leptons, nucleons and $\gamma$-rays resulting from top-down'' (non-acceleration) models for the case of uniform and isotropic source distribution. We explored two different classes of top-down'' scenarios: the wide earlier investigated class of X particles coming from collapse and/or annihilation of cosmic topological defects (such as cosmic strings, monopoles, etc.) and the models of super-heavy long-living X particles with lifetime of the order or much greater than the current Universe age.
 Physics , 1998, Abstract: We propose a formula for flux of extremely high energy cosmic rays (EHECR) through decay of superheavy particles. It is shown that EHECR spectrum reported by AGASA is reproduced by the formula. The presence of EHECR suggests, according to this approach, the existence of superheavy particles with mass of about $7 \times 10^{11}$GeV and the lifetime of about $10^9$ years. Possibility to obtain a knowledge of $\Omega_0$ of the universe from the spectrum of EHECR is also pointed out.
 Physics , 2000, Abstract: Increasing interest towards the observation of the highest energy cosmic rays has motivated the development of new detection techniques. The properties of the Cherenkov photon pulse emitted in the atmosphere by these very rare particles indicate low-cost semiconductor detectors as good candidates for their optical read-out. The aim of this paper is to evaluate the viability of solar panels for this purpose. The experimental framework resulting from measurements performed with suitably-designed solar cells and large conventional photovoltaic areas is presented. A discussion on the obtained and achievable sensitivities follows.
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