On the Anisotropy of Galactic Cosmic Rays
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 {g}c(z,p,μ ), 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 {g}s{(z,p,μ )\propto (p| μ | )}η {sgn}(μ ) with η = 2 - s, where s denotes the power-law spectral index of interstellar turbulence. Second, the interstellar Compton-Getting anisotropy {g}c(z,p,μ )\propto {H}cμ is independent of momentum and linearly proportional to {H}cμ . 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 (g\propto | μ | {sgn}(μ )) 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2019
- DOI:
- 10.3847/1538-4357/ab24c1
- Bibcode:
- 2019ApJ...879...29S
- Keywords:
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- cosmic rays;
- diffusion;
- ISM: magnetic fields;
- turbulence