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

¹ Istituto per la Scienza e Tecnologia dei Plasmi (ISTP), Consiglio Nazionale delle Ricerche, Via Amendola 122/D, 70126 Bari, Italy
e-mail: oreste.pezzi@istp.cnr.it
² The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, UK
³ Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
⁴ Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Teknikringen 31, 11428 Stockholm, Sweden
⁵ Dipartimento di Fisica, Università della Calabria, Ponte P. Bucci, Cubo 31C, 87036 Arcavacata di Rende, CS, Italy
⁶ Istituto Nazionale di Astrofisica – INAF, Direzione Scientifica, Roma, Italy
⁷ Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA

Received: 22 November 2023
Accepted: 1 March 2024

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 confining particles and lead to fast particle energization. These structures may also modify the properties of the turbulent, nonlinear transfer across scales.

Aims. 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.

Methods. 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.

Results. 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%). 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.