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

TL;DR

This study uses high-resolution simulations to explore how expansion and turbulence influence the evolution of magnetic flux ropes in the solar wind, revealing key factors controlling their size and structure during propagation.

Contribution

It introduces a simplified 2D model with turbulence to analyze the internal dynamics of flux ropes, highlighting the roles of expansion, turbulence, and plasma beta in their evolution.

Findings

Radial size of flux ropes increases due to spherical expansion.

Turbulence mainly affects the transverse structure of flux ropes.

The ratio of internal to propagation timescales influences flux rope expansion.

Abstract

Coronal mass ejections (CMEs) represent the most extreme solar products, showing 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 constant velocity in the spherically expanding solar wind, employing the expanding box model, which allows for high spatial resolution. Our setting is simplified, with uniform and non-magnetised solar wind, to which we superpose turbulent fluctuations. We find that the spherically expanding geometry alone perturbs the flux rope equilibrium, producing a radial head-tail velocity profile and a radial size increase. The ratio between the expansion and Alfv\'en timescales, associated respectively to propagation and internal crossing time, 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, spreading 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, suggesting that the ratio between internal and propagation timescales might help in better classifying different kinds of radial expansions for flux ropes.