The arrival directions of Galactic cosmic rays (CRs) are highly isotropic.
This is expected from the presence of turbulent magnetic fields in our Galactic
environment that repeatedly scatter charged CRs during propagation. However,
various CR observatories have identified weak anisotropies of various angular
sizes and with relative intensities of up to a level of 1 part in 1,000.
Whereas large-scale anisotropies are generally predicted by standard diffusion
models, the appearance of small-scale anisotropies down to an angular size of
10 degrees is surprising. In this review, we summarise the current experimental
situation for both the large-scale and small-scale anisotropies. We address
some of the issues in comparing different experimental results and remaining
questions in interpreting the observed large-scale anisotropies. We then review
the standard diffusive picture and its difficulty in producing the small-scale
anisotropies. Having set the stage, we review the various ideas and models put
forward for explaining the small-scale anisotropies.Comment: 60 pages, 16 figures; invited review for Progress in Particle and
Nuclear Physics (PPNP
