Simulating Cosmic-Ray Diffusion Coefficients in the Presence of Synthetic Magnetic Turbulence with a Scalable Compressive Component: First Results for Heliosheath Parameters

Galactic cosmic-ray (GCR) modulation studies have suggested that the ratio between the parallel and perpendicular mean free paths (MFPs) of these particles is a key parameter governing the observed, steep increase in high-energy GCR intensities within the heliosheath. Here, GCR diffusion coefficients are computed for heliosheath turbulence through direct numerical test-particle simulations, using a grid-based Fourier-noise model for a turbulent magnetic field with an arbitrary fractional longitudinal (compressive) magnetic variance, which is introduced. In the case of purely compressive turbulence, the parallel MFP does not converge, suggesting free streaming along the background field. For levels of compressibility more in line with observations in the inner heliosheath, including significant isotropic as well as anisotropic transverse components, parallel diffusive behaviour is regained, with a strong dependence on compressibility. Perpendicular MFPs increase with energy, in line with theoretical expectations. Intriguingly, the computed ratios of parallel to perpendicular MFPs do not greatly differ from those expected in the inner heliosphere, but are sensitive to increases in compressibility. Finally, the addition of a compressive component to a turbulent field with a transverse variance, which is held constant, results in enhancements to both parallel and perpendicular transport, implying that compressive turbulence cannot be neglected when modelling particle transport coefficients in this region.