The anisotropy of multi-TeV cosmic rays

Abstract

The arrival directions of cosmic rays will be isotropized by the deflection of these charged particles in the Galactic magnetic fields. For example, a 10 TeV proton in a typical Galactic field of 2 micro Gauss has a gyroradius of only 0.005 parsec (=1000 AU) which is much smaller than the distance to any postulated sources. However, observations of TeV cosmic rays by Milagro, Tibet III, ARGO, and IceCube, show anisotropies on both large and small angular scales. These observations require the detection of large numbers of cosmic rays because the anisotropies are less than a few parts in 1000. The large angular scale anisotropies, such as a dipole, could point to diffusion from a nearby source, but the smaller scale anisotropies of extent ~10 degrees are much more difficult to explain. Possibilities that have been explored in the literature include magnetic funneling of cosmic rays from nearby sources and acceleration by magnetic reconnection in the heliosphere's magnetotail. No matter what the mechanism, these observations provide new information about cosmic ray production, nearby magnetic fields, and how the cosmic rays observed at Earth are affected by their propagation.