Superdiffusion of Cosmic Rays: Implications for Cosmic Ray Acceleration

TL;DR

This paper investigates how superdiffusion caused by magnetic field line divergence in MHD turbulence affects cosmic ray propagation and acceleration, revealing altered diffusion behaviors and implications for shock acceleration efficiency.

Contribution

It introduces a model of superdiffusion in cosmic rays due to magnetic field line divergence, impacting their acceleration in shocks and highlighting the importance of local magnetic field effects.

Findings

Superdiffusion causes perpendicular displacement to scale as x^3 or x^{3/2}.

Superdiffusion reduces acceleration efficiency in perpendicular shocks.

Efficient acceleration occurs with small-scale magnetic fields in pre-shock regions.

Abstract

Diffusion of cosmic rays (CRs) is the key process of understanding their propagation and acceleration. We employ the description of spatial separation of magnetic field lines in MHD turbulence in Lazarian & Vishniac (1999) to quantify the divergence of magnetic field on scales less than the injection scale of turbulence and show this divergence induces superdiffusion of CR in the direction perpendicular to the mean magnetic field. The perpendicular displacement squared increases, not as distance $x$ along magnetic field, which is the case for a regular diffusion, but as the $x^{3}$ for freely streaming CRs. The dependence changes to $x^{3/2}$ for the CRs propagating diffusively along magnetic field. In the latter case we show that it is important to distinguish the perpendicular displacement in respect to the mean field and to the local magnetic field. We consider how superdiffusion changes the acceleration of CRs in shocks and show how it decreases efficiency of the CRs acceleration in perpendicular shocks. We also demonstrate that in the case when small-scale magnetic field is being generated in the pre-shock region, an efficient acceleration can take place for the CRs streaming without collisions along magnetic loops.