The effects of expansion and turbulence on the interplanetary evolution of a magnetic cloud

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
² Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Via del Fosso del Cavaliere 100, 00133 Roma, Italy

^⋆ Corresponding author: mattia.sangalli@unifi.it

Received: 16 March 2025
Accepted: 13 May 2025

Abstract

Coronal mass ejections represent the most extreme solar products, and show complex and dynamic structures when detected in situ. They are often preceded by a shock and carry a magnetic cloud organised as a flux rope, surrounded and permeated by turbulent fluctuations, and whose radial size expands during propagation. We investigate the internal dynamics of the 2D section of a cylindrical flux rope propagating at a constant velocity in the spherically expanding solar wind. To do this, we employed the expanding box model, which allows for high spatial resolution. We used a simplified setting with a uniform and non-magnetised solar wind, on which we superpose turbulent fluctuations. We find that the spherically expanding geometry alone perturbs the flux rope equilibrium and produces a radial head-tail velocity profile and a radial size increase. The ratio between the expansion and Alfvén timescales, which are associated to propagation and internal crossing time respectively, controls the resistance to transverse stretching and the increase of the flux rope radial extent; the plasma beta controls the overall size of the structure. Turbulent fluctuations mainly affect the flux rope transverse structure and spread its axial field at distances comparable to its size; on the contrary, dynamics along the radial direction remains coherent and the increase in radial size is still consistently observed. We validate our results by comparison with statistical observations and dimensionless estimates, such as the expansion parameter and the radial size scaling exponent. Such a comparison suggests that the ratio between internal and propagation timescales might help in better classifying different kinds of radial expansion for flux ropes.