A COARSE-GRAIN ANISOTROPY STUDY OF COSMIC RAYS ABOVE 10 17 EV

When averaged over large solid angles, the cosmic ray intensity measured by the Fly's Eye reveals no statistically significant anisotropy. The sky is divided into 6 quadrilateral regions of equal solid angle which are centered on the directions of the galactic coordinate system's axes. For each of nine energy bands, we compare the number of detected cosmic rays in each region with the number expected from an isotropic intensity. Plots show the percentage deviations together with the statistical uncertainties. Introduction. Patterns in the arrival directions of ultra -high energy cosmic rays could provide information about the locations of the cosmic ray sources and/or properties of the magnetic field in our part of the Galaxy. An isotropic extragalactic particle population would produce an isotropic intensity within the Galaxy. This is a consequence of Liouville's theorem and requires no specific knowledge of the magnetic fields in the Galaxy ( Hillas 1972). On the other hand, if most ultra-high energy cosmic rays originate in the Galaxy's disk, a magnetic field of finite extent should not mask the anisotropy of the cosmic ray source distribution. The anisotropy should be salient if the Larmor diameter of particle orbits is comparable to, or greater than, the disk thickness of the Galaxy's magnetic field. Taking the magnetic field to have a mean strength of 2.2 ìG and a thickness on the order of a kpc, this condition is satisfied for protons with energies above 1018 eV. The analysis method used in this report is based on the galactic coordinate system and goes beyond looking for a galactic latitude dependence in the cosmic ray intensity. The method is meant to facilitate comparison of the experimental results with anisotropy expectations derived from any model. We divide the entire sky into 6 (mutually exclusive) lobes of 2π/3 steradians each. The centers of the lobes are the 6 directions defined by the 3 axes of the galactic coordinate system (cf. Figure 1). In terms of galactic latitude and longitude coordinates ( b, � ), these six directions are the following: