Fast, Stable, and Physical: Hyperbolic, Magnetic Field-Aligned Diffusion in SPH

TL;DR

This paper presents a novel implementation of magnetic field-aligned hyperbolic diffusion in SPH, demonstrating improved stability, accuracy, and speed for simulating diffusive processes in magnetohydrodynamics.

Contribution

It introduces the first field-aligned hyperbolic diffusion method for SPH, including linear-exact gradients and reconstruction, enhancing stability and accuracy in MHD simulations.

Findings

LESPh with reconstruction grows the magneto-thermal instability (MTI) while SPH does not.

Both LESPH and SPH remain stable with aligned diffusion.

LESPh converges faster and is more accurate in the L1 error norm.

Abstract

In this paper, we introduce the first implementation of magnetic field-aligned hyperbolic diffusion for standard smoothed particle (magneto-)hydrodynamics (SPH), and its linear-exact gradient extension (LESPH). Hyperbolic diffusion differs from traditional parabolic methods by incorporating the physical characteristic speed of diffusing particles and is computationally faster. This work extends it to encompass field-aligned diffusion, linear-exact gradients, and linear reconstruction to limit dissipation. Several standard test problems are presented: a diffusing slab, diffusion around a ring, a Gaussian pulse, and the magneto-thermal instability (MTI). The MTI only grows for for LESPH with reconstruction, and not for SPH. Both LESPH and SPH remain stable while fully aligning diffusion to magnetic fields. LESPH is more accurate and converges faster in the L1 error norm. SPH and LESPH both see improvements when using when also using linear reconstruction. These methods apply to other diffusive transport such as cosmic rays, viscosity, or magnetic resistivity.