Evolution of Interplanetary Coronal Mass Ejection Complexity: a Numerical Study Through a Swarm of Simulated Spacecraft

Coronal mass ejections (CMEs) are the main source of adverse space weather in the inner heliosphere. These large-scale transients, characterized by intense and highly-twisted magnetic field bundles, often drive fast-forward interplanetary shocks and turbulent sheaths, and contain prolonged periods of southward pointing magnetic field. The interaction of CMEs with other interplanetary structures and other CMEs can drastically alter their global and local properties during propagation, and increase their complexity. In-situ measurements carried out by spacecraft in radial alignment are critical to advance our knowledge on the evolutionary behavior of CMEs and their magnetic structures during propagation. Yet, the scarcity of radially aligned CME crossings restricts investigations on the evolution of CME magnetic structures to a few case studies, preventing a comprehensive understanding of CME complexity changes during propagation. In this study, we perform numerical simulations of CMEs interacting with different solar wind streams using the linear force-free spheromak CME model incorporated into the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model. The novelty of our approach lies in the investigation of the evolution of CME complexity using a swarm of radially aligned, simulated spacecraft. Our scope is to determine under which conditions, and to what extent, CMEs exhibit variations of their magnetic structure and complexity during propagation, as measured by spacecraft that are radially aligned. Results indicate that the interaction with large-scale solar wind structures, and particularly with stream interaction regions, doubles the probability to detect an increase of the CME magnetic complexity between two spacecraft in radial alignment, compared to cases without such interactions. This work represents the first attempt to quantify the probability of detecting complexity changes in CME magnetic structures by spacecraft in radial alignment using numerical simulations, and it provides support to the interpretation of multi-point CME observations involving past, current, and future missions.