The composition and energy spectrum of primary cosmic rays (PCRs) are the only observables at high energies to study the nature of sources accelerating PCRs to 1000 TeV. These observables have been ...directly measured up to ∼300 TeV with detectors aboard balloons and satellites. But measurements at >1000 TeV have to be obtained indirectly from ground-based observations of extensive air showers. However, their interpretation relies on an inadequate knowledge of hadronic interactions at 1000 TeV. The GRAPES-3 experiment is designed to operate at 30 TeV providing a sizable overlap in energy with direct measurements, enabling the selection of a suitable model of hadronic interactions at ∼1000 TeV. We present salient features of GRAPES-3 including details of muon multiplicity distributions observed with a 560 m2 detector as a function of shower size from an analysis of data of 545 days. These distributions were compared with expectations from Monte Carlo simulations, using some of the hadronic interaction generators in CORSIKA, to deduce energy spectra of five nuclear groups in the 100-1000 TeV region. A comparison of GRAPES-3 results with direct measurements indicates that SIBYLL provides a good description of hadronic interactions for interpreting our data. These measurements extend energy spectra and composition of PCRs that is consistent with extrapolation of direct measurements.
Aims. We seek to estimate the average level of MHD turbulence near coronal mass ejection (CME) fronts as they propagate from the Sun to the Earth. Methods. We examined the cosmic ray data from the ...GRAPES-3 tracking muon telescope at Ooty, together with the data from other sources for three closely observed Forbush decrease events. Each of these event is associated with frontside halo coronal mass ejections (CMEs) and near-Earth magnetic clouds. The associated Forbush decreases are therefore expected to have significant contributions from the cosmic-ray depressions inside the CMEs/ejecta. In each case, we estimate the magnitude of the Forbush decrease using a simple model for the diffusion of high-energy protons through the largely closed field lines enclosing the CME as it expands and propagates from the Sun to the Earth. The diffusion of high-energy protons is inhibited by the smooth, large-scale magnetic field enclosing the CME and aided by the turbulent fluctuations near the CME front. We use estimates of the cross-field diffusion coefficient $D_{\perp}$ derived from the published results of extensive Monte Carlo simulations of cosmic rays propagating through turbulent magnetic fields. We then compare our estimates with the magnitudes of the observed Forbush decreases. Results. Our method helps constrain the ratio of energy density in the turbulent magnetic fields to that in the mean magnetic fields near the CME fronts. This ratio is found to be ~2% for the 2001 April 11 Forbush decrease event, ~6% for the 2003 November 20 Forbush decrease event and ~249% for the much more energetic event of 2003 October 29.
The change in the spectral index from about
-
2.7
to
-
3.1
at
∼
3
×
10
15
eV in the all-particle energy spectrum of primary cosmic rays is very significant for learning about the nature of cosmic ...sources of ultra-high energy particles and their acceleration and propagation in the galactic disk. Any observation of a fine structure in the spectrum would be important for improving our understanding of these physical processes. The GRAPES-3 air shower array has been designed to achieve higher precision in determination of various shower parameters to enable observation of any fine structure in the energy spectrum, if it exists. The details of the shower detectors, shower trigger and the data acquisition system are presented here along with estimates of trigger efficiencies from Monte Carlo simulations for primary photons (
γ
-rays) and several nuclei.
Certain aspects of ultra-high energy (UHE) cosmic ray astrophysics require correlated studies on the electron and muon components of air showers, namely, the search for cosmic ray sources through
γ
...-ray astronomy and studies on the variation of the nuclear composition of primary cosmic rays with energy. While studies on the electron component provide basic information about the arrival direction and energy, it is the muon component that plays a crucial role in distinguishing primary
γ
-rays from charged cosmic ray particles and in determining the composition.
A large area (560
m
2
), tracking muon detector of the GRAPES-3 experiment, operating at Ooty in southern India, has been designed for detailed studies on both of these aspects of UHE cosmic ray astrophysics. We present here the details of the muon detector, associated electronics and the data acquisition system. A brief discussion of the potential of the muon detector is presented through simulation studies.
The absence of a well established point source of very high energy
(
≳
10
TeV
)
γ-rays in the sky, makes the measurement of the angular resolution and the absolute pointing accuracy of an extensive ...air shower (EAS) array a challenging task. In the past, several groups have utilized the reduction in the isotropic flux of cosmic rays due to the shadows of the Moon and the Sun, to measure the angular resolution and the absolute pointing accuracy of their arrays. The data collected from the GRAPES-3 EAS array, over the period of 4 years from 2000 to 2003, has been used to observe the shadow of the Moon at a level of
∼
5
σ
and that of the Sun at a lower level of significance. The high density of the detectors in GRAPES-3 enabled an angular resolution of 0.7° to be obtained at energies as low as 30
TeV. The angular resolution studies were further extended by using two other techniques, namely, the even–odd and the left–right methods. All three techniques have yielded nearly identical results on the energy dependent angular resolution.
The GRAPES-3 extensive air shower (EAS) array started operation with 256 scintillator detectors at Ooty in India. Each detector is viewed by a fast photomultiplier tube (PMT) mounted at a height of ...60
cm above the scintillator. However, for further expansion of the array, an alternative readout of the scintillator using wave-length shifting (WLS) fibers is employed. This resulted in improved performance with a larger photon signal and a more uniform response. With the inclusion of a second PMT, the dynamic range for particle detection has been increased to ∼5
×
10
3 particles m
−2. We now use plastic scintillators, developed in-house to cut costs. The measurement of the density spectrum, shows a power law dependence with an index
γ
=
1.57
±
0.04. Using the zenith angle dependence of the density spectrum, an attenuation length Λ
a
=
98–106
g
cm
−2 for the EAS is obtained. These measurements are found to be consistent with the results reported earlier by other groups.
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