The search for cosmic-ray anisotropies

Abstract

At the present time there is no generally accepted evidence for any statistically significant anisotropy in the energy range 1017-1019 eV. The upper limits on the possible anisotropy provide strong evidence that these particles are extra-galactic. In that part of the cosmic-ray magnetic rigidity spectrum below ca . 2 x 1011 V the interplanetary magnetic field effectively prevents the detection of anisotropies in interstellar space and the only isotropies measured are associated with the solar wind and its associated magnetic field. In the range of magnetic rigidities extending from 1011 to 1012 V the cosmic-ray intensity shows evidence for a small anisotropy of about 2 x 10~4 which can be explained as the result of solar motion relative to the average galactic rotation in our neighbourhood. When this is removed the residual deviations from the mean intensity preclude any systematic sinusoidal variation greater than 2 x 10~4. This high degree of isotropy is most easily understood if these particles are members of an extra-galactic population and it is suggested that this extra-galactic component predominates from the highest cosmic-ray energies down the spectrum at least as far as ca . 1011 V rigidity.