Particle scattering in turbulent plasmas

High-energy particles stream during coronal mass ejections or flares through the plasma of the solar wind, causing instabilities that lead to wave growth at specific resonant wave numbers, especially within shock regions. This paper presents results on particle transport and scattering in turbulent plasmas with excited wave modes, employing a hybrid simulation code that treats heliospheric turbulence with an incompressible magnetohydrodynamic approach while separately addressing a kinetic particle description. Additionally, a semi-analytical model based on quasilinear theory (QLT) is compared to numerical findings, aiming for a deeper understanding of pitch-angle scattering coefficients. The calculations indicate a good agreement between particle simulations and QLT for broad-band turbulent spectra; however, for higher turbulence levels and particle beam-driven plasmas, the QLT approximation becomes less accurate. The resonance gap at µ = 0 poses a significant challenge for QLT in steep turbulence spectra, while test-particle computations reveal no issues in particle scattering across this region. The discrepancies arise from the oversimplification of sharp resonant wave-particle interactions in QLT, which do not account for the broader resonances observed in test-particle calculations due to nonlinear effects. These findings emphasize the crucial implications for both numerical simulations and analytical particle transport methodologies.