Modelling electron beams in flux-rope coronal mass ejections

Counterstreaming suprathermal electron beams are frequently observed within interplanetary coronal mass ejections (CMEs) and are commonly interpreted as signatures of particle trapping along closed magnetic flux ropes. Their properties are expected to depend on the internal magnetic structure of the CME flux rope and on the level of particle scattering. We investigate how the internal magnetic configuration of flux-rope CMEs, particularly magnetic twists, influences the propagation, focusing, and mirroring of trapped electrons. Our objective is to determine the conditions that influence counterstreaming electron beam formation at 1 au. We employed the Flux Rope in 3D (FRi3D) CME model implemented in EUHFORIA to simulate CMEs with a realistic magnetic field structure. Electron transport was modelled with the test-particle code PARADISE by solving the focused transport equation along CME field lines. We varied the magnetic twist of the flux rope and the parallel mean free path of the electrons to quantify their influence on beam formation and evolution. The simulations produce pronounced counterstreaming electron signatures at 1 au. The modelled flux ropes contain three mirroring regions, arising from the enhanced magnetic field strength near the flux rope's footpoints and apex. Clear counterstreaming beams are reproduced for (łambda_∥ ≳ 10) au . As the magnetic twist increases, the signatures remain but become broader and more asymmetric. The values of (łambda_∥) required to reproduce these signatures are substantially larger than typical values for the ambient solar wind but remain consistent with the comparatively ordered large-scale magnetic structure expected inside flux ropes. Our results show that the properties of suprathermal counterstreaming electron beams are highly sensitive to the internal structure of flux-rope CMEs. In particular, magnetic twist and pitch-angle scattering jointly control the degree of focusing, mirroring, and asymmetry observed at 1 au. This supports the use of suprathermal electron pitch-angle distributions as diagnostics of flux-rope structure in CMEs.