The influence of different turbulence models on the diffusion coefficients of energetic particles

Abstract

We explore the influence of turbulence on the transport of energetic particles by using test particle simulations. We compute parallel and perpendicular diffusion coefficients for two‐component turbulence, isotropic turbulence, a model based on Goldreich‐Sridhar scaling, noisy reduced magnetohydrodynamic turbulence, and a noisy slab model. We show that diffusion coefficients have a similar rigidity dependence regardless which turbulence model is used, and thus, we conclude that the influence of turbulence on particle transport is not as strong as originally thought. Only fundamental quantities such as particle rigidity and the Kubo number are relevant. In the current paper we also confirm the unified nonlinear transport theory for noisy slab turbulence. To double check the validity and accuracy of our numerical results, we use a second test particle code. We show that both codes provide very similar results confirming the validity of our conclusions.

Explaining Tev Cosmic-Ray Anisotropies With Non-Diffusive Cosmic-Ray Propagation

“…However, even in these regions the strength of magnetic turbulence is typically smaller than that of the guiding field, so the coherent field may still play an important role (Gaensler et al 2011). This can be seen in studies of pitch-angle scattering in large interstellar coherent magnetic structures (Barge et al 1984;Desiati & Zweibel 2014) and in studies of parallel and perpendicular diffusion (Giacalone & Jokipii 1999;Tautz et al 2013Tautz et al , 2014Hussein et al 2015;Shalchi 2015). We plan on incorporating these cross-fieldline transport effects into our framework in the future.…”

Simulations of energetic particles interacting with nonlinear anisotropic dynamical turbulence

“…This type of turbulence description is supported by observations in the solar wind (see, e.g., Matthaeus et al 1990, Osman and Horbury 2009a, Osman and Horbury 2009b, Turner et al 2012, turbulence simulations (see, e.g., Oughton et al 1994, Shaikh and Zank 2007 as well as analytical treatments of turbulence (see, e.g., Zank and Matthaeus 1993). More details concerning the used model can be found in the aforementioned articles or in the corresponding diffusion theory papers (see, e.g., Hussein et al (2015) and Hussein & Shalchi (2016)).…”

“…It should be emphasized, however, that nonlinear effects can be important for parallel diffusion and non-resonant interactions can influence the diffusion parameter in certain parameter regimes (see Shalchi 2009 for a review). Spectral anisotropy can also have an effect but this effect is weaker than originally thought (see Hussein et al 2015). For perpendicular diffusion, however, the details of the turbulence seem to be less important because the perpendicular diffusion coefficient depends only on the so-called Kubo number and the parallel diffusion coefficient (see Shalchi 2015).…”

The influence of the Kubo number on the transport of energetic particles

“…For small Kubo numbers, however, completely different results can be obtained. In this small Kubo number regime, test-particle simulations have been performed in order to test different nonlinear transport theories and the results they provide (see [26,31]). It was shown that only UNLT theory works in this regime whereas NLGC theory and its extensions presented in the literature completely fail.…”

“…For short parallel mean free paths, we also expect that the perpendicular mean free path is short [31]. Therefore, in the limit of strong pitch-angle scattering, we can omit the term k^k 2 in the denominator of equation (39).…”