Turbulence and particle energization in twisted flux ropes under solar-wind conditions

Abstract

The mechanisms regulating the transport and energization of charged particles in space and astrophysical plasmas are still debated. Plasma turbulence is known to be a powerful particle accelerator. Large-scale structures, including flux ropes and plasmoids, may contribute to confining particles and lead to fast particle energization. These structures may also modify the properties of the turbulent, nonlinear transfer across scales. We aim to investigate how large-scale flux ropes are perturbed and, simultaneously, how they influence the nonlinear transfer of turbulent energy toward smaller scales. We then intend to address how these structures affect particle transport and energization. We adopted magnetohydrodynamic simulations perturbing a large-scale flux rope in solar-wind conditions and possibly triggering turbulence. Then, we employed test-particle methods to investigate particle transport and energization in the perturbed flux rope. The large-scale helical flux rope inhibits the turbulent cascade toward smaller scales, especially if the amplitude of the initial perturbations is not large ($ 5<!PCT!>$). In this case, particle transport is inhibited inside the structure. Fast particle acceleration occurs in association with phases of trapped motion within the large-scale flux rope.