The turbulent evolution of an interplanetary magnetic cloud in the expanding solar wind

Interplanetary coronal mass ejections often carry large-scale magnetic clouds, which display internal substructures and small-scale fluctuations. These complex multi-scale clouds represent the main drivers of geomagnetic storms at Earth, and the amplitude, coherence and variability of their magnetic field all contribute to their geoeffectivity.

We present high resolution simulations of a magnetic cloud interacting with turbulent fluctuations while propagating in the spherically expanding solar wind; we investigate the effects of turbulence on the internal dynamics and magnetic field variability. Our simulations employ the expanding box model, a semi-lagrangian numerical approach that allows to follow the evolution of a parcel of plasma in the spherical solar wind flow, decoupling the small-scale internal dynamics from the large-scale motion.

We recover observed features such as the radial expansion of the structure and the low-temperature and low-beta signatures of magnetic clouds, together with a quite rich internal dynamics. We also find that turbulent reconnection and field transport produce smaller secondary magnetic flux ropes, possibly enhancing the cloud's geoeffectivity; this behaviour might also account for the relatively small magnetic correlation lengths which have been estimated in interplanetary magnetic clouds.