Simulating cosmic-ray transport with adiabatic focusing

Abstract

Context. Magnetized space plasmas such as the heliosphere contain a spatially variable guide magnetic field that is superposed by turbulence. Understanding the transport of energetic particles in such an environment is important for many applications. Aims. The modifications of the scattering mean free path caused by the effect of adiabatic focusing have so far been studied only analytically. Here, the exact analytical conditions are reproduced in a numerical simulation to allow for a detailed comparison. Methods. A Monte-Carlo simulation code was modified to include a guide magnetic field with a constant focusing length. The turbulence strength was kept constant either throughout the simulation space or relative to the variable background field strength. Results. In contrast to both quasi-linear and nonlinear analytical predictions, an increased parallel mean free path is found for almost all cases. Particle diffusion is found to be asymmetric depending on the initial particle velocity relative to the guide field. Conclusions. The increased parallel mean free path can be explained by a coherent particle motion toward lower magnetic field strengths. The possible reduction of the diffusion parameter is consequently obscured. © 2012 ESO.