Particle scattering in turbulent plasmas with amplified wave modes

TL;DR

This study investigates how amplified wave modes in turbulent plasmas affect particle scattering, comparing hybrid simulations with quasilinear theory to improve understanding of particle transport in solar wind conditions.

Contribution

It provides a detailed comparison between numerical hybrid simulations and quasilinear theory for particle scattering in turbulent plasmas with excited wave modes, highlighting the limitations of QLT.

Findings

Good agreement between simulations and QLT for broad-band spectra

QLT's accuracy decreases at high turbulence levels and for steep spectra

Test-particle results show particles can cross the resonance gap at μ=0

Abstract

High-energy particles stream during coronal mass ejections or flares through the plasma of the solar wind. This causes instabilities, which lead to wave growth at specific resonant wave numbers, especially within shock regions. These amplified wave modes influence the turbulent scattering process significantly. In this paper, results of particle transport and scattering in turbulent plasmas with excited wave modes are presented. The method used is a hybrid simulation code, which treats the heliospheric turbulence by an incompressible magnetohydrodynamic approach separately from a kinetic particle description. Furthermore, a semi-analytical model using quasilinear theory (QLT) is compared to the numerical results. This paper aims at a more fundamental understanding and interpretation of the pitch-angle scattering coefficients. Our calculations show a good agreement of particle simulations and the QLT for broad-band turbulent spectra; for higher turbulence levels and particle beam driven plasmas, the QLT approximation gets worse. Especially the resonance gap at $\mu=0$ poses a well-known problem for QLT for steep turbulence spectra, whereas test-particle computations show no problems for the particles to scatter across this region. The reason is that the sharp resonant wave--particle interactions in QLT are an oversimplification of the broader resonances in test-particle calculations, which result from nonlinear effects not included in the QLT. We emphasise the importance of these results for both numerical simulations and analytical particle transport approaches, especially the validity of the QLT.