On Simplified Numerical Turbulence Models in Test-particle Simulations

TL;DR

This paper discusses the limitations of conventional superposition methods for creating isotropic turbulent magnetic fields in test-particle simulations, highlighting issues with physical constraints and their impact on simulation results.

Contribution

It introduces the challenge of constructing physically consistent isotropic turbulence models and analyzes the influence of magnetic divergence and wave vector isotropy on simulation outcomes.

Findings

Conventional superposition methods cannot produce perfectly isotropic, divergence-free turbulent magnetic fields.

Magnetic field strength significantly affects simulation results, unlike divergence or wave vector isotropy.

Non-zero divergence and anisotropic wave vectors have limited impact on particle scattering and diffusion results.

Abstract

Using the conventional approach of superposing plane waves, it is not possible to create a strictly isotropic turbulent magnetic field structure that obeys all physical constraints, which are (i) equal mean of all magnetic field components; (ii) isotropy of the wave vectors; and (iii) vanishing divergence of the magnetic field. Such magnetic fields are widely implemented in test-particle Monte-Carlo simulations, which are used to obtain (i) scattering mean free paths of charged particles; (ii) field line diffusion coefficients. It is shown that, while the turbulent magnetic field strength plays an important role for the results, such does not seem to be the case for a non-zero magnetic field divergence and/or the isotropy of the wave vectors.