Evolution of magnetic cloud signatures in the turbulent solar wind: virtual spacecraft analysis

Interplanetary coronal mass ejections (ICMEs) carry magnetic clouds, multi-scale structures which span a considerable fraction of an astronomical unit and display a rich dynamics at many spatial scales, including turbulence. Spacecraft that encounter an ICME can measure smoothly rotating "magnetic cloud" (MC) intervals or less organised "magnetic obstacle" (MO) ones.

We aim to understand to what extent the interplay of expansion, turbulence, and internal cloud dynamics affects the magnetic cloud properties, which then translate to signatures measured by spacecraft. We perform 2.5D MHD simulations of a magnetic flux rope embedded in the turbulent expanding solar wind, using the expanding box model, which decouples large and small scales and provides high resolution. We employ virtual spacecraft to probe the local plasma properties.

The flux rope exhibits a coherent large-scale expansion, and clear and stable MC signatures are always found by spacecraft intercepting the flux rope core. Disordered MO signatures appear at the flux rope edges, due to both expansion and turbulent transport. The strength of the expanding flow controls the angular extent of coherent signatures, whereas the intensity of turbulence controls the variability between different spacecraft encounters and the amount of distortion and deflection that the cloud experiences. Our results support the idea that the MC/MO dualism is a consequence of the impact geometry. The presence of MO signatures at the edges is instead controlled by the initial confinement of the axial flux rope field by magnetic tension: disordered signatures disappear for narrow flux ropes.