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

Abstract

Context. 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 confine particles and lead to fast particle energization. These structures may also modify the properties of the turbulent nonlinear transfer across scales. Aims. We investigate how large-scale flux ropes are perturbed and, simultaneously, influence the nonlinear transfer of turbulent energy towards smaller scales. We then address how these structures affect particle transport and energization. Methods. We adopt magnetohydrodynamic simulations for perturbing a large-scale flux rope in solar-wind conditions and possibly triggering turbulence. Then, we employ test-particle methods to investigate particle transport and energization in the perturbed flux rope. Results. The large-scale helical flux rope inhibits the turbulent cascade towards smaller scales, especially if the amplitude of the initial perturbations is not large (~5%). 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.