Cosmic Rays in Interstellar Turbulence

Summary

The acceleration, transport and loss of cosmic rays in the galaxy are determined primarily by their interactions with the turbulent interstellar electromagnetic field. Collisions with other particles are very rare, although they affect the abundances of rare species through spallation. The observed high degree of isotropy and temporal and spatial homogeneity are a consequence of rapid motion along the field and the scattering of the cosmic-ray particles by turbulent magnetic-field irregularities, which causes spatial diffusion. The basic equation governing the cosmic-ray transport is the Parker transport equation, which has survived stringent tests by in situ spacecraft observations in the heliosphere. Because of our lack of knowledge of the parameters and boundary conditions, only relatively crude solutions have been discussed. These allow an approximate determination of the diffusion coefficients. Comparison with observation suggests strongly that the cosmic rays can diffuse across the magnetic field much more rapidly than in classical diffusion. The physical mechanism for this is discussed.

Introduction

Cosmic rays are very fast charged particles which are accelerated to high energies by plasma processes, principally collisionless shock waves, occuring in astrophysical plasmas. The acceleration at collisionless shock waves relies on the interaction of the charged particles with turbulence, which causes spatial diffusion both along and perpendicular to the magnetic field. This allows some of the particles to cross the the shock many times, to gain many times their original energy.