On the Anisotropy of Galactic Cosmic Rays

Abstract

Abstract In the interstellar medium at rest, containing low-frequency magnetohydrodynamic linearly polarized slab Alfvén waves, the anisotropy of relativistic galactic cosmic rays consists of two parts: the streaming anisotropy g s (z, p,μ), caused by the spatial gradient of the isotropic part of the cosmic ray distribution function, and the interstellar Compton–Getting anisotropy , caused by the momentum gradient of the isotropic part of the cosmic ray distribution function. Both anisotropies depend differently on the cosmic ray pitch-angle cosine μ, cosmic ray momentum p, and cross-helicity state H c of the Alfvenic slab turbulence. First, the streaming anisotropy is independent from H c and varies as with η = 2 − s, where s denotes the power-law spectral index of interstellar turbulence. Second, the interstellar Compton–Getting anisotropy is independent of momentum and linearly proportional to . These different pitch-angle dependencies can be tested by the Liouville mapping technique to infer the pristine interstellar cosmic ray anisotropy from measurements inside the solar system. For cosmic rays with energy of 4 TeV the derived pristine interstellar cosmic ray anisotropy suggest the linear ( ) pitch-angle dependence. This is well explained by the interstellar Compton–Getting anisotropy, provided the Alfvén speed in the local interstellar medium is about 62 km s−1.