Cosmic-Ray Transport Theory in Partially Turbulent Space Plasmas With Compressible Magnetic Turbulence

Recently, a new transport theory of cosmic rays in magnetized space plasmas extending the quasilinear approximation to the particle orbit has been developed for the case of an axisymmetric incompressible magnetic turbulence. Here, we generalize the approach to the important physical case of a compressible plasma. As previously obtained in the case of an incompressible plasma, we allow arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field. For the case of quasi-stationary and spatially homogeneous magnetic turbulence we derive, in the small Larmor radius approximation, gyrophase-averaged cosmic-ray Fokker–Planck coefficients. Upper limits for the perpendicular and pitch-angle Fokker–Planck coefficients and for the perpendicular and parallel spatial diffusion coefficients are presented.