Turbulence and Diffusive Transport of Cosmic Rays in the Very Local Interstellar Medium

Abstract

We study the transport of fast charged particles, such as galactic cosmic rays, in the very local interstellar medium (VLISM), which is currently being explored by the two Voyager space probes. Guided by the observations of magnetic fluctuations, the paper develops a simple theoretical framework for computing scattering rates and spatial diffusion coefficients that can be used to model cosmic-ray transport in the VLISM. The local interstellar magnetic turbulence is represented as a superposition of (a) Alfvénic, (b) transverse 2D, and (c) longitudinal components obeying distinctive geometry rules in the plasma frame. The model is based on the weakly nonlinear formalism where particle trajectory’s deviation from the unperturbed helix is caused primarily by guiding-center diffusion across the mean magnetic field. The transverse component plays the dominant role in perpendicular diffusion, while the longitudinal component has only a minor effect. Pitch-angle scattering is extremely weak in the VLISM, so that cosmic-ray transport can be considered essentially scatter-free on heliospheric scales. We test our theoretical model with the help of particle orbit simulations to find good agreement for perpendicular diffusion. We also find that cosmic rays disperse faster than in a conventional random walk (diffusive) process if the turbulence power spectrum contains fluctuations whose wavelength is larger than the size of the heliosphere.