Scattering of the UHECR at small pitch angle by damped plasma waves

Abstract

In spite a lot of theoretical and experimental effort that has been achieved in ultra-high energy cosmic ray (UHECR) scattering research in last few decades, some questions remain unanswered, or partially answered. Two of them, that will be in the focus of this paper are: possible source of UHECRs and the acceleration mechanism of cosmic rays beyond PeV energies. Small pitch-angle scattering of UHECRs and possible confinement has been investigated using quasilinear theory in order to analytically calculate pitch-angle Fokker-Planck coefficient. CR particles resonantly interact with oblique low frequency damped waves. We show that the resonance function is broadened due to damping effects and this result is compared with the nonlinear broadening. Unlike the case of purely parallel (or antiparallel) propagating waves in slab turbulence, the presence of the compressive magnetic field component of oblique fast-mode waves allows the cosmic ray particles to resonantly interact with these waves through the n = 0 resonance, together with gyroresonance, which strongly influence the Hillas limit. The derived results can be used to compute the parallel mean free path for all forms of the turbulence spectrum; it has been applied on the transport and propagation of CRs close to ultra-high energies in the Galaxy. An accurate understanding of particle acceleration in astrophysical sources could help to interpret eventual transition from Galactic to extragalactic origin of cosmic rays, if any, and the shape of the UHECR spectrum at the highest energies.