Evolution of Magnetic Complexity and Coherence within Interplanetary Coronal Mass Ejections: Novel Insights from Multi-Point Observational and Numerical Investigations

Many aspects of the three-dimensional (3-D) structure and evolution of coronal mass ejections (CMEs) in interplanetary space remain unexplained. A prominent question is whether they are magnetically-coherent objects, and at which scales such a coherence exists. Recent studies also highlighted the ever-changing nature of their magnetic complexity during propagation, primarily as a consequence of interactions with other large-scale solar wind structures. In this presentation, we discuss the following aspects of CME magnetic complexity and coherence: what is the spatial distribution of magnetic complexity within CMEs? Across what spatial scales might CMEs behave as magnetically coherent objects? How do complexity and coherence depend on the heliocentric distance and the specific evolution history of a CME?

First, we will present results from a statistical analysis of complexity changes affecting the magnetic structure of interplanetary CMEs observed by multiple spacecraft in radial alignment at heliocentric distances of Mercury, Venus, and 1 au between 2008 and 2014. The consideration of a statistical set of CMEs allows us to generalize previous observational results based on CME case studies and to draw, for the first time, conclusions on the frequency, causes, and effects of magnetic complexity changes on CME structures. Complementing observational insights, we will then present a numerical investigation of how complexity and coherence evolve as a consequence of CME interactions with different solar wind streams, performed 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 and coherence using a swarm of ~20000 simulated spacecraft, which allows an augmented investigation of the above questions, from an in situ perspective as consistent as possible to actual multi-spacecraft observational efforts. Such investigations highlight the key role of interactions with other large-scale solar wind structures in shaping the complexity and coherence of CMEs, yielding insights into the evolutionary paths of CMEs during propagation, and paving the way towards improved predictions of CME properties at 1 au.